URGH, this post is a mess... First, the Wii U is considerably more powerful than PS360 consoles, not "on par with" or "underpowered" next to them. It could have any of the games you mentioned. It could've had Unreal Engine 4, too. Epic, however, decided not to put it there - There's an important distinction to make here, and it's this... WON'T DO IT doesn't mean CAN'T DO IT. In many cases, "third party" publishers and development houses wouldn't bother for various reasons, but it wasn't impossible. Difficult? In some cases, certainly. Perceived as not worth one's while, or too much of a risk, and therefore the motivation to do it is little to zero? That's also possible. But impossible? There are solid reasons to believe that this isn't the case. One might also say that, ultimately, Nintendo could've done more for the Wii U, too. PS360 consoles couldn't handle Xenoblade Chronicles X or LOZ: Breath Of The Wild. The scale and performance of those games are beyond their capabilities, and that's before getting to the fact that it's doing some things better than X4 titles on less meatier specs, or even bringing the GamePad into play. Still, hardly anybody ever takes into account the fact that Nintendo prioritises stable 60FPS performances over photo-realistic visual presentation - On here, one can find screenshots of games on other platforms, but what those screenshots never tell you is that the game isn't going for the same frame rate; In the case of the PS4, the visual element often takes prevalence, and 30FPS is seen as "enough" (See Drive Club, then Forza 5, for example). "Third party" endeavours were largely unoptimised, and the Wii U wasn't a lead platform. So, the fact that Wii U versions held up in those capacities at all is rather a testament to that console. I could list a number of things that the Wii U did in terms of performance, and show you the receipts to back up my comments, but perhaps I could do that elsewhere (because I find that NeoGAF, for a large part, is obtuse as fuck, tbqh).
With regard to the Switch's capacity, there has been a lot of misinformation, from the chip that powers it (which isn't a Tegra X1, btw - This much is a fact that has been confirmed on multiple occasions from the collective horse's mouth (that is, by Nintendo, Nvidia and "third party" developers) since the Switch reveal, before and after launch, backed up with word-specific evidence stretching from October to this month), to a lack of robustness in reporting widely available information which is critical to this topic, to talk of "significant downgrades" (a term that has been mentioned only on here, and not once by Nintendo's partners, without evidence to back that narrative). The great tragedy of your discussion is that any desire to pursue the truth has been superseded by one on the part of some antis and certain gaming publications to draw fast conclusions, however untrue, and paint the bleakest possible picture. There has been a general acceptance, despite evidence before people's eyes which contradicts their proposals. There's this thing where some make out that the Switch is meeting its full potential at launch, when no console ever does that. There's a pattern here, because exactly the same thing happened with the Wii U; NSMBU wasn't the height of the Wii U's abilities, and neither was Batman: AC: AE, or Assassin's Creed 3. We saw Bayonetta 2, The Wonderful 101, Mario Kart 8, XCX, SSB4 and LOZ: Breath Of The Wild. While on the other side of this discussion, you have disillusioned fanboys who hoped that "NX" would be a "Polaris Powered, Gimmick-Free Third Goliath" to Nintendo's David-esque predecessors - Also, See how a potentially decent discussion on accurate Foxconn leaks turned into a Tumblr fanfic.
Regarding your question of ports, LOZ: Breath Of The Wild wasn't optimised by Nintendo for the Switch, yet is has more stable frame rates and a better resolution than the Wii U version, which the PS360 consoles couldn't handle. So, the simple answer to that is "Yes, it could have excellent ports of last gen titles". I'm surprised that this is even a question, given that Rime (which was a PS4 exclusive previously) is there, and The Elder Scrolls V: Skyrim: Special Edition was confirmed for the Switch in its reveal clip. There have been reports of multiple PS4 projects being moved (likely ported) to the Switch - This is about the pursuit of truth, and the "significant downgrades" crowd (once more, that is people on here, not developers) must answer this: Why would multiple PS4 projects be moved to the Switch, if it meant that they would have to be downgraded significantly? Surely an XBox One version would be less work, or less risk, if their proposals held water at all? Does this not sound bizarre!?
So, what is the Switch's actual capacity? If one is looking purely at numbers, then the Switch will appear to be underwhelming. If one is comparing to the PS4 (which is the leading console, and probably the lead development platform), then it shouldn't be a surprise that the PS4 has the better performance, especially when Switch versions of non-exclusives are neither from-the-ground-up or optimised/tailor-made for it. If one is less concerned with numbers, and looks at real-life world performance, then it becomes very apparent that the Switch as a home console is in the ballpark of the XBox One, give or take a little, and therefore able to have the non-exclusive games on it and the PS4 - A lot of people forget that the XBox One is less powerful than the PS4, and doesn't always match it, too, but this is fine for the Switch, because the ability to play on the move is a key differentiator and a welcome trade-off. To get certain publishers on board, this was a pre-requisite!! In portable mode, it is more capable than the Wii U, but it has a more modern feature set, so, it could still have those titles. In the worst case scenario, it could be 720p versions docked and undocked, but that would be fine. Even the XBox One has 720p titles, but unlike the Switch, it confines you to a TV set and four wall space. I realise that this post will go against the tide of popular narrative, but it's still the most accurate.