Hypothetically, when you're found to infringe a patent for a product that was already deployed, then the damage was already done and migrating at that point won't save you ;-)
Right but this clause assumes you are the one suing facebook for patent infringement and unless your patent is along the lines of "we use react.js to do x", then you probably have the flexibility to port it to a different framework.
Hypothetically if you ever grow so big and frighten facebook to that degree, migrating off react would be an easy problem to solve.