TLDR the game engine is ass.
See as mentioned the engine is the problem. Bungie has in the past admitted that the engine is a pain in the ass. It's also a modified Destiny 1 engine. It's also clear by this that the engine is not capable of creating the experience Bungie truly wants. Hence they do need to go back to the drawing board and develop a new engine capable of doing what they need it to do. I mean create a new engine too. Not just modify yet again the old engine that is the problem. They don't even need to make a Destiny 3 for this to happen. Just build the new engine and import the old content or rebuild it and eventually just release a new client.
I have a different theory about this. Jason Schreier is the guy who reported about Destiny engine problems, he's also the guy who wrote the article on Bethesda saying that engines don't need to be entirely rewritten from the ground up, but can be upgraded progressively.
Destiny has an masterpiece of an engine compared to Bethesda. Of course none of us can measure the feasibility of fixing Destiny's engine issues, but I don't think it's impossible to overhaul its filesystem.
This is the contradiction: they tell us they are focusing on D2, rather than 3, but all they are showing is that they have no intention of overhauling the engine.
IN FACT they are telling us they need to remove half the content because they don't have a way to fix the game. The filesystem isn't being touched, it's just cut content.
This seems to have a rather straightforward explanation: if D2 can't be fixed at a deep level, and has to be maintained by cutting content, then it means that their best programmers aren't "focusing" on D2, they abandoned it so they can work on actual new things that will take years to materialize.
The fact we have no D3, and that D2 is untouched, is precisely the proof their technical team is busy elsewhere, working on stuff that won't surface for years. In the meantime there's only the same old D2 that needs to keep the cash flow open.
P.S.
Also consider this: if it's true we're getting in D2 content built for D3, then it means D3 was built as almost identical, or it would take a massive amount of work to backport assets to a completely different engine.