You have to take into account that FFXIII was a major negative for the FF brand as a whole. Yes, it's true that it was received badly and had lousy word of mouth -- but both of these things have also reduced the sales potential of FFXV.
I thought for a while that the combination of FFXIII and FFXIV 1.0 had permanently damaged the brand, but I think it's had some recovery in recent years. First off there's FFXIV ARR, which is probably the most impressive recovery any game has ever had post-launch. It's pretty much the only B2P+monthly fee MMO that's successfully landed since WoW. Secondly, Squeenix has kept the FF franchise in the public consciousness by re-releasing almost the entire franchise over the past few years (1-9 on mobile, 10+10-2 on console, various ports to handheld). They've also had several games that kinda just look back fondly on the series, basically doing PR via games for the franchise (specifically thinking of FF Record Keeper on mobile and Theatrhythm on 3DS).
I agree with you that they haven't had a mainline title land with a bang in a long time, but I think that FFXV might be different. Looking at where the Tomb Raider franchise was a couple years ago to where it is now, I think that FFXV might be an equivalent to the TR reboot.
http://kotaku.com/5224724/tomb-raider-lifetime-sales-show-off-lara-crofts-biggest-hits
http://www.ign.com/articles/2015/04/06/tomb-raider-reboot-has-sold-85-million-copies
The franchise was basically never able to have a game sell more than 3M after the PSX era, but the TR reboot is on its way to selling 10M. I don't know if FFXV can mimic that success in that same time period, but I think over a couple years it has a good chance of hitting those numbers.
I think FFXV might be the Gundam Wing of the FF franchise, in that they'll able to make inroads internationally that have eluded them before. They're making a lot of decisions that make the game appealing to a mainstream fanbase (open world, no turn based combat, ~~dreamy~~ cast of characters). And contrary to the potential sales shrinking due to negative reviews of previous games, I think that they have a much larger potential fanbase than the previous games due to geekdom/anime culture becoming much more mainstream and anime-ass-anime not being strictly consumed by either children or hardcore otaku culture. I'm comparing to the late 90's/early 00's when FF was at its peak in terms of quality and sales, and anime was either saturday morning cartoons by Dic or weekday shows by Toonami, or else imported on VHS from the uber-hardcore. Argument can be made that that was already changing by the time FFXII came out what with the Adult Swim lineup, but I really don't think that compares to how mainstream it's become with Crunchyroll/Funimation/Hulu/Netflix in addition to many casual/non-tech savvy fans easily able to pirate via sites that stream manga/anime fantranslations, compared to when it was necessary to torrent at relatively uncommon internet speeds for VHS rips.
I think that there's a ton of teenagers with a casual interest in anime, maybe they've only seen Free and a few other shows - I think that FFXV is going to appeal to that market, which I don't think really existed to a major degree when FFX or FFXII came out. I think FFXV is going to be a lot of people's first FF game ever.
I also think that their crossmedia marketing push may have a some minor positive impact in mainstream sales just because of the people involved with it (Aaron Paul, Lena Headey, Sean Bean), but I usually have a pretty pessimistic view on the effectiveness of crossmedia promotions and don't really see FFXV's being the exception to it. However, I do think that it shows that Squeenix is going to invest a HUGE amount of money marketing the game, and that's going to lead to some sales.
MGSV is a bit above a 5 million seller worldwide, so yes, I do think it will perform similarly to that. If it stalls out on 5-6 million that really doesn't get it close to the number we're talking about.
I think that if it stalls at 5-6M after the first few months, they've still got a good chance to hit their goal within the generation. I think that the PC release has a floor of ~300,000 and a ceiling of ~1.5M, and depending on the quality of the port/game, and the time it takes to release that port, I think it can land pretty much anywhere in that range. That puts them at ~5-7M within the first 12 months of release.
Again, if the PS4K comes out this year, I think that FFXV will be one of the games most heavily promoted by Sony, and will also likely have have a bundle attached to it. I think that on the high-end, that can lead to and additional 1M sales, or at least boost the legs of the game via people wanting a game to show off the PS4K.
That puts it at ~6-8M. If it's at the higher end of that range, I think it's pretty inevitable that they'd be able to hit 10M by the end of the generation just through sales on steam/PSN/etc.
Here is a list of active console AAA franchises that did 10+ million on their last outing: GTA, Fallout, Skyrim, Borderlands, Call of Duty, Battlefield, Battlefront, Destiny. Exactly two of those are real RPGs -- they're by the same publisher and fine-tuned to a far more general-audience-friendly design and theme than FF would bring to the table. Most of them are shooters. When you get outside of Bethesdaworld you pretty much need extremely strong support from the PC market in order to even make it to the next tier down from there, and SE is explicitly giving PC gamers sloppy seconds at best here.
When you look at this analysis, you need to accept three things to start: 1) that even huge mainstream success AAA franchises like Assassin's Creed aren't 10 million sellers today in 2016, 2) that Final Fantasy as a brand and a product offering has never been as popular as even some of those sub-10m franchises, much less the ones I listed above, and 3) the FF brand has been on a decline for the past decade and nothing has happened that suggests a reversal of that trend.
Those are all games that hit 10M+ pretty much instantly at launch, which I've never claimed FFXV had any chance at all of doing (they definitely don't). I'm arguing that the game definitely has a realistic path to hitting that number by the end of the generation, if the following happens:
1) FFXV gets very positive reviews (85-92 metacritic range)
2) Embraced by the traditional FF fanbase (based on the FF uncovered hype and reaction, I think that's already happened). I think positive word of mouth is important, but I think what's even more important is avoiding a negative reaction from fans, who vocally discourage anyone from buying the game (which is what happened with 13 and both its sequels). I think it's hugely important that FFXV avoids that.
3) If the PC port comes out within 2-4 months from console release, and is a good port, it can sell close to ~1M over a few months. If it comes out 6-8 months late, I think it will still generate ~1M over 12 months, but many of those being at 25-50% off.
4) If the PS4K comes out this year, FFXV will get a PS4K patch and be heavily marketed by Sony in addition to being bundled with the game, generating an additional ~1M over a few months
5) If all of the above happens, they'll be able to hit 10M this generation from PSN/Steam/retail sales
You are really misreading the hype.
I really just don't see this as being super outlandish. They would need all the cards to line up in their favor, which may be unlikely (particularly unsure about the PS4K timing) but I think that the path to 10M over ~5 years is definitely there.