I would think that advertisers would eventually notice anyway. If lead quality decreases due to poor ad targeting or high-value users leaving the platform then they'll start pulling their advertising dollars back.
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".
Advertising is an old boy's club. They spend money based on personal relationships, brand image, reputation, reliability, and trust.
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.
I can't see advertising being part of the long term plan here. He's a payments guy, and there is much more money in turning Twitter into an economy in of itself. This would be tricky if he needed to rely on banks etc, but more recently, he is also a crypto guy. It will be interesting to see how creative he gets in that area, but I would think that his dream would be to create a fluid online economy. I would expect him to first attempt reinventing the concepts of cash, and loose change, but in a digital platform, rather than one's pocket.
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".