Legislating that everyone must always be safe regardless of what app they use is a one-way ticket to walled gardens for everything. This kind of safety is the rationale behind things like secure boot, Apple's App Store, and remote attestation.
Also consider what this means for open source. No hobbyist can ship an IM app if they don't go all the way and E2E encrypt (and security audit) the damn thing. The barriers of entry this creates are huge and very beneficial for the already powerful since they can afford to deal with this stuff from day one.
Doesn't have to be a law. Can just be standard engineering practice.
Websockets for example are always encrypted (not e2e). That means anyone who implements a chess game over websockets gets encryption at no extra effort.
We just need e2e to be just as easy. For example maybe imagine a new type of unicode which is encrypted. Your application just deals with 'unicode' strings and the OS handles encryption and decryption for you, including if you send those strings over the network to others.