It's even a problem with allies as well as adversaries; countries can potentially get around their own legal limitations by agreeing to spy on one another, and where needed to build a case, covering their tracks via parallel construction. Whether officially or unofficially, any backdoor keys would leak almost immediately.
Not just governments, but various mafias, several corporates, random masonic offshoots, and the occaisional creative individual presumably have backdoors into all sorts of stuff. This isn't about creating backdoors, this is about being allowed to use them within the public legal framework.
China already does require backdoors of both local Chinese firms and foreign firms with HQs in China, although the requirement may not be "technically official" (it can certainly be interpreted that way from their 2017 law, though).
I think it's disgusting how supposed "democracies" have been trying to emulate China, both in terms of surveillance and censorship. UK is one of the worst offenders here -- sometimes they didn't even hide the fact they were using China as a role model.
There used to be a time when the U.S. government and other countries would condemn China for this sort of stuff.
https://en.wikipedia.org/wiki/Five_Eyes
https://en.wikipedia.org/wiki/Parallel_construction