One might reasonably assume the "bad guys" they're trying to catch would go elsewhere though, if they have any sense. So then you're just left with innocent people to spy on.
What if catching the stupid bad guys just means the smart bad guys take their place? Like a spray that kills 99% of bacteria, all you're potentially doing is applying a selective pressure towards being more technically smart.
And in this case, being more technically smart might just mean clicking the link to the E2E encrypted web chat site rather than the server-to-client encrypted site. Perhaps, though, the government will start banning websites that offer E2E encrypted chat, and require hosting companies to not let you host such apps yourself.
Those are who they want to spy on anyway.
For serious criminals, terrorists etc they have other tools, banning commercial encryption wont help with those...