Maybe they don't think it's as likely as you think. Or maybe they think the time horizon is long enough that it is worth it. Whatever it is, I assure you they are not too stupid to realize this is a possibility. They just differ on either the degree of the risk, or on the value they get in return. In which case, logic dictates that if you believe the actual risk is higher than they would tolerate, to find a way to convince them of that.

That's a matter of facts rather than values. The "impossible promise" is that a private company can keep a kilobyte of data out of the hands of every spy agency on Earth for any length of time.

Why does this concern apply to the French intelligence agency but not, say, the code signing certificate for Whatsapp?

On one level it does apply to certificates.

On another level, pushing a compromised update to every phone in Europe (to gain access equivalent to having a copy of the backdoor) would be noticed by the publisher, and the binary difference would quickly be found by security researchers.

And yet the immense majority of the population accept & use messaging platforms operated by "not France" where the operator would be able to obtain everyone's key with a couple clicks. (Yes, this includes Apple's).

That makes sense because it has been brought about as a consequence of consumer preference. It wouldn't make sense for it to have been required by the government.

Unfortunately, last time I heard, consumers actually dictate what the government cares about.

I think they think when that happens they'll change it. Or they'll change it on a schedule... "Just use HTTPS" is good enough for every website and web app on the planet (including ones that handle more messages than the entire country of France) but not a state intelligence agency can't keep their TLS keys secret?

"Just use HTTPS" sends French messages directly to NSA, insofar as those platforms are concerned. In that case there's not even a copy of any encryption key...

France is giving everything they receive to the NSA anyway, so there's not much difference.