The growing disappointment in Xbox is a combination of four things:
1) Halo is the flagship Xbox franchise, and 343 look like they're mucking it up again. There's a bad vibe building around the game; the kinda one that people felt around Crackdown 3.
2) Xbox Series X needed to avoid the Xbox One mistakes in order to have a fighting chance in NA and UK. They're making the same Xbox One mistakes all over again (i.e. no games, confusing messaging, directionless leadership) so the people who fantasized about an Xbox return to the top are realizing that their fantasy just might not come true. Crazier things have happened, but all of the indicators so far point to yet another generation where you're not missing out if you skip the Xbox console.
3) Philip Spencer and crew have been yammering for years in front of any and all live microphones about the great games (e.g. greatest games lineup ever yearly mantra) the Xbox has. The truth is starting to set in that Philip and crew have been all smoke no fire. A lot of people have been saying (rightfully so) that by just being quiet and not appearing in every mp3 and mp4 file on the internet, Philip and crew would not create unrealistic expectations around Xbox's games library and it's supposed superiority over past Xbox libraries. I guess people are starting to question that if the current and future Xbox has this supposedly unstoppable superior games library then why are Xbox, yet again, relying on a bunch of oldass 15 year old+ games to sell their new box? It's one thing to have backwards compatibility with new flagship games, it's another to have backwards compatibility with no new big games.
4) Whether you like it or not, the console space has three players: Sony, Nintendo, and Microsoft. Each company will, whether justly or not, be routinely compared to the other companies in the space. With that in mind, assuming no consoles are delayed, Xbox is going up against PlayStation in a few months. The former has leadership that has been all talk and no action, while the latter has leadership that has been (almost to their own detriment at some points) no talk and all action. So the "outrage" is less about the unimpressive Halo (and if we're being honest, even more unimpressive Xbox library) and more about the fact that one company seems to keep talking and getting it wrong while the other seems to keep shutting up and getting it right. If you're invested in the company that keeps fucking up then you're probably going to be "outraged" at what's going on with that company.
To be honest, I don't see a lot of genuine "outrage." Disappointment and dislike, yes, but genuine anger? Not really. I think what you're seeing is a lot of diehard Xbox fans coming to the realization that the current Xbox leadership has overpromised and underdelivered one too many times.