It would make for a good alternative, but so would any app that doesn't implement scanning. If they can force Signal to implement client-side scanning or ban it

It would make for a good alternative, but so would any app that doesn't implement scanning. If they can force Signal to implement client-side scanning or ban it entirely, they can do the same to PGP software. In theory you could do RSA by hand, but it would require a lot of hard work.