But federation will definitely be handing it to them on a silver platter.

Not really because all those big web companies already have crawlers already anyway. It's literally more work for them to support open standards than it is not.

Plus they already have 100 million Threads users. They don't care about the 2mil Mastodon users.

The real concern is that federating with Threads might create network effects that pull Fediverse users more and more into just using Threads

"I can't follow all the cool entertainment accounts from here, so I follow them on Threads instead." <-- more likely.

Another concern I’ve seen mentioned is, a lot of instances have rules against advertising.

Threads cannot make other instances run ads.

And finally, Facebook is just a garbage corporation.

Facebook is one of the biggest contributors to OpenStreetMap and makes lots of open source software. I can say with 100% confidence that you've used software by Facebook even if you never ever visited any Facebook/Meta service. Just because parts of a big corp suck bad, doesn't mean that everything they make sucks.

I find it so hilarious that so many freak out about stupid Facebook, yet fucking Truth Social by Trump (which is literally a Mastodon instance full of neo-nazis) is barely blocked by anyone.