The company I work for writes software for larger companies that I could see getting into a patent battle with Facebook. Wouldn’t this clause make such an effort much more difficult and expensive?
Also, for small startups that can’t even afford to waste time writing their own version of react, won’t this clause be a big motivator to switch to preact or vuejs? Ultimately, whether you could win in court is a moot point. If Facebook even threatens to sue, as a small company who’s worth less than half of the lawyer fees you need to even respond, you’re dead. Like you said, dumping react is way easier than risking such a nightmare. When paper tigers require expensive lawyers to prove they are paper tigers, they are liable to come to life, are they not?
