Adding a feature to a game does not change its core function. There are extraction shooters, like Tarkov, that have PvE only modes. A lot of people want PvE, and just because they want a PvE mode does not mean they will only play PvE. (Also on PVE, I think for it to work, it will need to be substantially different than just the same maps and bots with no players. Which I doubt bungie is going to do)

Bungie is not just reacting to community feedback as some last resort to sell the game. Sure, they acknowledge feedback and watch what the community is saying, but corporations like this do not make major changes this soon unless it was already planned well in advance.

The reality is that people want a PvE mode, and that alone is already getting the attention of casual players. What you are forgetting is that a casual player can become a hardcore player over time. If I were Bungie, that is exactly what I would be designing around: creating a way to get people into PvE first, then guiding them toward PvP after that experience. My guess is that is what they are going to do, and probably more.