I understand not wanting to sack him after a 66 points/5th place finish, but they should still have been prepared for the possibility of a mid-season change before a ball was kicked this season. This kind of a total collapse was nearly impossible to envision, but alarm bells were still ringing to a considerable degree about Ange's long-term suitability for the job. 1 point from 5 matches after that Chelsea match, 4 match losing streak near the end of the season, struggles in breaking down low blocks and relying on last minute goals and comebacks to win against inferior teams on many occasions. These cannot simply be brushed away as hindsight.
United, despite him having a strong first season and winning a cup in his second season as well, pulled the plug on Ten Hag after 9 league matches, sitting with 11 points. We had 13 from our first 9. In a spell where we gave away a 2 goal lead against Brighton, and dropped 5 points against Leicester and Palace. Two weeks later, on Nov 10, we lost to another relegation fodder Ipswich at home. If they actually had a replacement plan ready, this would have been a great time to pull the plug on him. Everything to play for in the league, no injury excuse or any other mitigating factors in sight. Instead they put all their eggs in Ange basket, an injury crisis started appearing on the horizon, and the rest is history.
Even if they no longer believe in him, which might not even be the case by the way, we are in purgatory now. Probably too late in the context of the season to replace him with a permanent long-term successor, but also too early to throw the towel by going with Mason. If they ever plan on taking that route again that is.
Between him, Mason, and a rushed appointment from a limited number of available candidates, I can't see an easy way out. Pick your poison.