All that to say that a ban doesn't have to be 100% effective to make a meaningful difference.
That's probably a crime in UK. It is a crime in plenty of countries.
Anyway, the most impactful an anti-e2e law can be is to force people into getting some functional thing from free-droid, instead of naively getting it from the play store. The bar of intelligence required for that is still pretty low.
Firstly, you can just rotate the key if that happens. It's one software update away.
Secondly, protecting keys isn't that hard. That's what HSMs are for. Not only have no secret keys ever leaked from the NSA as far as I know, not even when insiders turned against them and leaked as much as they could, but this isn't a noteworthy achievement either.
Plus, who would be stupid enough to use that protocol? It is sending bright flashing messages saying "we're reading your emails, mate!". Only people who were legally compelled to use WhatsApp would be reachable, everyone else would more to some other system.