At the end of the day, Destiny 2 is a sequel and not DLC because Bungie is marketing, packaging, and selling it as a sequel instead of DLC. Therefore it is, by definition, a sequel. Is it the most ambitious sequel ever made? Nah, doesn't look to be. But not every sequel is a complete overhaul. Sometimes you get the jump from Assassin's Creed to AC2, and sometimes you get the bump from 2 to Brotherhood. Both types of sequels are valid, and both have their place. That one is a far bigger change doesn't make the other less of a sequel, it just makes it a more evolutionary approach.
Snd let's be honest, what this is really about isn't labelling Destiny 2 a sequel or DLC, what this is really about is money, specifically the $60 Destiny 2 will cost as a sequel instead of the $40 people think it would have cost as DLC. Except that was never going to be the case. We've had 4 Destiny expansions now: two at $20, one for $40, and the final $30 DLC. Looking across these it's not hard to see the tiers at play in terms of what your money buys. Even if you accept the argument that Destiny 2 isn't adding anything that The Taken King didn't already add, there difference in scale is obvious. TTK added one patrol zone, 8 missions to the main campaign (with admittedly another 10 or so extra missions wrapped up in various quests), 3 or 4 strikes (1 PS4 exclusive) with a 5th added for free 6 months later, and a raid. Destiny 2 is adding 4 planets/patrol zones which, while not shown, are claimed to be considerably larger than in Destiny), what sounds to be around twice the story missions, and presumably 6+ strikes. By the standards clearly already laid out whether you call that an expansion, DLC, or a sequel that's $60 worth of content.
As for what we've seen thus far that couldn't be done as DLC, quite simply the entire opening mission. Not one Destiny expansion has undone anything that's come before. Nothing has been replaced or eliminated (with the sole exceptions of replacing Dinklebot with Nolandroid and organizing Destiny 1's missions into quests while leaving the missions themselves unchanged). They've expanded only. Look at the Plaguelands. A good third of that area is recycled from the Cosmodrome, yet the Cosmodrome wasn't replaced. Instead the Plaguelands are a separate area with all the changes restricted to them while at any point I can return to the original Cosmodrome which is identical to how it was on launch. Expand only, remove nothing. I can role a new character today and get almost the identical experience as on launch day (again the only duuiifferences being the quest structure and Nolandroid), meanwhile the Warlock I rolled on launch day can still replay any mission he wants. He can still go to the tower, but Bungie wants to blow the tower up. How do you get rid of the Tower in an expansion while still allowing the game to be played by new players? Plus after 2 1/2 years of balance patches, archetypes, metas, etc. there's a LOT of cruft that'a built up. By pitching this as a sequel Bungie gets to start over from scratch, and they have. They didn't just blow up everyone's vaults, they completely redid the approach to weapon loadouts. The change from primary/special/heavy to kinetic/energy/power alone is something that couldn't be done as DLC, because such a fundamental weapon rearrangement would destroy the balance of the entire game. You would have to redesign every encounter in the game to compensate for it. Finally there's the backend, invisible stuff. We know Destiny's developer environment was a disaster and needed to be completely redone. I'd be shocked if fixing their dev tools didn't require changes to the engine to incorporate the new content that left it fundamentally incompatible with what was already there.