Advertisers will be back. They are just following each other and de-risking. This isn't the first time it's happened and there's been plenty of worse issues with Facebook and YouTube. The biggest issue* for Musk when it comes to advertising is that Twitter isn't actually good for advertising other than promoting brand messaging, with advertising spend going to reduce during the global recession it will hurt Twitter more than say Google Search.
*Assuming Musk will reign it in and hire a solid team to manage Twitter.
They are de-risking because they want to get ahead of twitter mobs calling them out online for enabling Musk with their advertising money. The only way they would ever return is if Musk suddenly becomes a normal, dignified person, which won't be happening. The advertisers will simply apply their money to other social networks instead.