As more of these are announced, fewer casual twitter lurkers will visit the site. Then it will become socially unacceptable for remaining brands to stay, and they'll leave too. Prolific twitter posters will find they no longer have any reach, so they'll just fade out.
Because it's not all advertisers leaving, there will still be ads just from less scrupulous companies. And because Elon is pushing the site to be more right wing, probably more right wing advertisers. Some users won't like that
It's traditionally conservative but more frequently has been endorseing candidates left of center since the right wing is batshit. They overall supported Obama, but frankly Obama is about as centrist as they come. There was a very long stint in the 00s and 10s where they would publish more conservative, hawkish pieces over any news to the contrary.
I doubt they’re pals. Their relationship seems to be very adversarial. Everyone hates musk and he sees them as a threat. Bezos is a neoliberal. Musk is a fascist. There is a difference, at least as far as optics are concerned.