I’m doubtful. Advertisers will prefer Threads. A massive install base in an app run by a company that has a proven track record of moderation that protects brands. Meta isn’t perfect but brands trust them.
Initially, I expect you're right, but advertisers also monitor whether users see ads, click on them, and make purchases once they click. Advertisers might not drop it as fast as users, but they will eventually go where the users go.