Didn't they take it all out because they wanted to update the game and all the stuff pre-Beyond Light wouldn't work. So they gutted the game. They'd have to redo all the old stuff to make it compatible again. They're never going to produce the vaulted stuff, it would cost too much time and money for no return. And that is a damn shame, it would have been nice if it was available to play from start to finish.
I disagree with the idea that the content would take too much time or effort to restore. That comes across to me as more horseshit excuses from Bungie. There are three major things I can think of that would break levels with an engine change:
- Lighting / VFX
- Bounding boxes / physics
- Scripted events
Lighting was likely restored because none of the other content looked any different after the update. This means the elements that likely presented the challenges were bounding boxes or scripted events... And yeah that's a fair bit of work, but so what? You have the original code and map data as reference, suck it up and do it. Bounding boxes may have needed to be re-flagged for their physics properties. Scripting could have needed a new language or to be re-compartmentalized into new call / storage methods (Bungie was vocal that game compiles / builds took too long at the time).
Neither of these is an unsolvable problem. They clearly had the resources to rework the content because according to them they did update half the content in the game during the development of beyond light. All the content that didn't hit the vault was reworked. Why couldn't they continue that work in the background during year 5, year 6, etc? It probably wouldn have gotten done by year 7 if they kept working at it. They probably just tested the waters with their vaulting announcement and people gave them a pass. It was simply cheaper to just pretend this content never existed.
There's also seasonal content forever removed from the game that was developed after the engine update. They could have put some effort into making that content playable again because it should be engine compatible. Being the assholes Bungie are though they not only removed that content from the game, they deliberately told youtubers they could not even document / archive it in video form. Destin used to make video documentaries of the seasonal stories to preserve them for furture players. Bungie shut that down.
I treat Bungie with the same level of respect they've treated their game / players / IP with, that being none. Bungie does not value their own work beyond pre-order money and first week sales. Never let them live what they did down.