I have to wonder if there was already some pushback from larger advertisers that prompted Elon's tweet in the first place. Time will tell but we've already got 4chan and some users and advertisers would rather go to a "mall-like" experience to not have to deal with the "crap".
The actual quality of the advertising product almost doesn't matter, it's not like any of the advertisers can reasonably independently validate the claims or performance, they're trusting them as given.
Elon has never been a member of the advertising club, he famously chooses to have his company refuse to even be a client, and he doesn't have a reputation for any of those values. Wouldn't at all be surprised if major clients put a hold on their Q4 spend with Twitter the moment they got news of the completed acquisition.
Google, Facebook, and Snap all had major advertising earning misses, the agencies would be pressuring for better deals regardless.
This isn't Bezos buying the Post outright and running it as a vanity project, Musk has stakeholders.
What you predict (financial ruin) will probably happen if he doesn't do that.