I'm not a lawyer, but you might have missed this clause in the patent grant:
> Notwithstanding the foregoing, if Facebook or any of its subsidiaries or corporate affiliates files a lawsuit alleging patent infringement against you in the first instance, and you respond by filing a patent infringement counterclaim in that lawsuit against that party that is unrelated to the Software, the license granted hereunder will not terminate under section (i) of this paragraph due to such counterclaim.
Also consider that most open-source licenses make no mention of patents at all, so you have no assurance that the company won't come after you for patents in their software.
Your example is solely if THEY initiate a lawsuit then you're allowed to respond, not if they do something against you that warrants suing them first.
Edit: E.g. They can copy your whole business, design and all, and if you're using React and sue them - then they can use that against you / you'd have to immediately cease using it; I'm not actually sure what the outcome would be if a license automatically expires.
Your example is solely if THEY initiate a lawsuit then you're allowed to respond, not if they do something against you that warrants suing them first.
Note that this is specifically regarding a patent assertion; i.e. the patent grant is not revoked if you bring legal action against Facebook for another reason, including trademark or copyright issues.
Edit: E.g. They can copy your whole business, design and all, and if you're using React and sue them - then they can use that against you
See above – this would specifically only apply to patents.
you'd have to immediately cease using it
This is not necessarily true – it just means that you lose your patent grant to React. There is no assertion here that Facebook hold any patents on React. It's obvious why this would be the case – if you bring a patent suit against Facebook, then they revoke any patent license they've granted you. I would expect any patent license – for open-source software or otherwise – to be the same.
Granted, this does nothing to affect the wider issue of a broken patent system.
> Notwithstanding the foregoing, if Facebook or any of its subsidiaries or corporate affiliates files a lawsuit alleging patent infringement against you in the first instance, and you respond by filing a patent infringement counterclaim in that lawsuit against that party that is unrelated to the Software, the license granted hereunder will not terminate under section (i) of this paragraph due to such counterclaim.
Also consider that most open-source licenses make no mention of patents at all, so you have no assurance that the company won't come after you for patents in their software.