I just think this is reductionist mentality because that could be said for litrelly any game.
Firstly there's a few things one the reason destiny 2 flat lined was because bungies poor management, destiny 2 took a major hit because years back they neutered a ton of the best talent and put it on Marathon so destiny 2 lost priority.
With better leadership if bungie was made to allocate 100% on destiny 3 you're running at full force, everything is all ideas... The idea that d2 is it is just silly because there's litrelly any story that could be told, new worlds a whole new saga, in gaming it can go anywhere.
D3 also would've brought new engine to make a good feeling game feel even that much better, a new game gets people excited again with new loot, new quests, new worlds, new possibilities, maybe they have a new mode that becomes fan favorites like iron banner, or trials etc..
To me, when someone says what exactly can d3 do that hasn't been done in d2, tells me they aren't and weren't invested in destiny. Because in that game the sky was the limit, d3 with a full on bungie is the difference. Destiny hasnt had priority and 100% attention in years and that's why it went on autopilot.
D3 would've absolutely been a hit.