I'm told that one reason companies might have chosen to avoid React was that they could have imagined themselves be in competition with Facebook at some point, and thereby in the danger zone with their old BSD licence.

They explicitly said in their FAQ that you could use React even in direct competition. None of this matters now, of course, because it's been relicensed. The only impact the patents license ever had on anyone was iff they initiated a patent lawsuit against Facebook.

The point is that if you're in direct competition, the chances of Facebook violating one of your patents is much higher, and then your cost of suing them is significantly increased (since you also have to rewrite your app).

If you're ok with trusting a direct competitor not to misbehave, then sure, use React under those terms while competing with FB. But large companies consider their patent portfolio to be a significant strategic asset, so it shouldn't be surprising that their legal counsel advises against accepting such a clause.

Would the previous license have become relevant if Facebook initiated a patent lawsuit against you?

Even being on the back-foot when having to defend your patents, by refactoring React out of your application, was apparently damning enough for people to migrate.

Yeah, obviously not all lawyers see eye to eye.