That's part of my argument. Let's assume we trust the Government of the UK to not abuse this (bear with me, I know it's a stretch...). So UK Govt, police, MI5 can read our messages but that's OK, because they're the "good guys". America sees this and enacts similar legislation, so they now have access to the back door. Hrm, Ok, but we're at least allies, so that's not too bad... Then China. Then Russia. Then a corrupt regime which is repressing its populace and arresting/killing dissidents. If you aim to allow the "good guys" access but not the "bad guys", you have to make moral judgements which companies are notoriously bad at.
Once you've hit that breadth of access, the backdoor isn't secure and the entire system is being snooped by, well, just about everyone.
As you say, anyone who is vaguely tech savvy will have a better, secure solution immune to these back doors.