We've already seen plenty of examples of that irl.
I'm sympathetic to that argument, but it seems to be employed very selectively by the tech industry.
We already rely on many backdoors of exactly that kind in form of mandatory auto updates. Not only is this seen as perfectly fine, it's widely regarded as a security best practice.
Why can Apple or Google or Microsoft manage to keep their signature keys secure for decades while any keys managed by a government agency would leak with mathematical certainty?
Also: who is to say that Apple, Google or Microsoft manage to keep their keys secret? Not all thieves would be stupid enough to tell, and nationstates tend to hold such advantages on ice until they have a good enough reason to use them.
And as some lower-hanging fruit: The repos of common programming languages and things like Docker Hub.
Python PIP, NodeJS NPM, Ruby Gems, we pull in a lot of stuff from people we don't even know. Every python project installs a gazillion of stuff from its requirements.txt. At least the OS updates come from a party we at least chose to do business with.
And it's not like this is not yet happening already. But I think it'll take a major Wannacry event before we'll stop doing this because it's just so damn handy.
But if you think of it, imagine you're coding and some random 'willywonka2586' on a public slack group says "Hey I wrote a handy library for that, here, go and install it and use it in a project for your customers!". This is kinda what we're doing.
It's not "perfectly fine"; but the alternative is millions of computers with known security vulnerabilities exploitable by anyone, which is far far worse than a potential backdoor used by large companies or governments.
At the very least, this would need some quantification of risk: Probability and impact of keys getting leaked vs probability and impact of not installing a backdoor.
Unfortunately, our beloved decisionmakers are gerontocratic, completely incompetent and bought out by corporate interests that also don't want encryption (e.g. copyright industry).
That X makes things worse for people in general just isn't strong evidence that governments will not do X. If you don't want X then you need to explicitly push back.
