> You would lose the legal right to use any patents which Facebook might (or might not!) have on React.

So a poker bluff that would be Russian roulette without an expensive comprehensive patent search (and researching their patents itself increases your liability; reading about all the ones that you later determine didn't cover React subjects you to triple damages if they covered any of your other tech).

Facebook could also have acquired existing patents at a later date from other companies and individuals and use them go after you if you triggered the revoke terms, so you had to search all patents ever, not just ones that Facebook filed or owned during your initial decision to use the library. You could argue you have to do that anyway, but keeping with the poker bluff analogy, Facebook could know of the patent already, have undisclosed terms that prevented React licensees from being sued by the holder, and have a pre-negotiated option to buy the rights during a licensee revoke event.