1. He overpaid by tens of billions. That is a phenomenal amount of money to lose on an unforced error.
2. Enough users, who produce enough content, have left to make X increasingly a forum for porn bots, scam accounts and political activists. It's losing its appeal as the place "where the news happens" and is instead becoming more niche.
3. The firings did not go well. X has struggled to ship new features and appears nowhere closer to the "everything app" Musk promised. It posts strange UUID error codes. The remaining developers seem to implement things primarily client side, to the extent I even wonder if they have lost their ability to safely roll out backend changes.
4. The capture of X by far-right agitators has led to long term brand damage for Tesla, Musk's most important business property.
I can't see any positive outcome from it.