Starfield wasn't the problem, halo infinite not having content at launch basically damaged the brand. You can argue making those types of games are expensive which makes sense but gamers expect fortnite level content
Really not sure one of those has to do with another.
If we're being completely honest with ourselves, Halo as a franchise has likely maxed out its potential. It's such an entrenched legacy IP that if you don't like Halo by now, you're not all that likely to suddenly get into it. And its existing fans want the game to be as close to the experience that they remember that you really can't change it up enough to appeal to players who don't like that type of shooter. Another example of that is Gears of War. That remake apparently didn't do numbers on PS5, and a good portion of it could be contributed to that type of game just not being appealing to audiences that aren't already playing it now.
Starfield was in a unique position that it was a new IP, so aside from it being from a known studio, it didn't have the weight of established franchise expectations, which is why they decided to take that exclusive. The unfortunate part is that this was the game where Bethesda found out in real time that the Bethesda Game template needs serious retooling.