What choice do they have? The company was hovering around profitable (2021 would’ve been without a lawsuit settlement) but that was before they were saddled with a ton of new debt. It’s possible that they could find new revenue models - that pay for checkmark scheme isn’t it but maybe a smart businessman could come up with a better variant - but that takes time and has to be done carefully since they have to keep the lights on in the middle. Driving away advertisers before finding that new revenue source doesn’t leave much time to iterate.

I don't think "should" is really a function here.

"Must" might be.

No one has figured out a way to monetize a social network without advertising, at least not at the scale Twitter has to operate.

If that's it's business model then sure, that's how it works.

A "de facto public square" would be public in conception, construction, and support from the start, which is one of the ways we know that Twitter is no such thing. Though it would likely also have some rules for how speech is/isn't conducted.

And all things considered, advertiser-friendliness is a sort of low-resolution but approximate passable democratic mechanism for marking boundaries of civilized discourse.

That depends on Musk’s goals. If his goal is to make money to pay off the massive amount of debt he gained as part of buying twitter, (as the attempted quick roll out of twitter blue would indicate), then yes he probably needs to care about what advertisers think. If he just wanted to leverage his position as one of the richest men in the world to ensure that twitter was a haven for free speech and screw the profitability, then no he wouldn’t need to be guided by advertisers. He probably can’t both want immediate returns on his investment and to quickly rock the boat though.

Television and radio pretty much works that way. Print media too.

In this case yes because you need to be profitable.