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If you say, "well yes but only for legal purposes", then the jig is up. End-to-end encryption must die. You can fiddle here and there with what getting the warrants and the wiretaps and all that looks like but you have decided the fundamental question. From then on, between civil libertarians and law enforcement the issue becomes a skrimage. The courts will hash it out and subsequent legislation will ebb and flow the limits on both sides. But you have decided the fundamental question. Humans are not allowed to actually have secrets. Period. That is what happens the second you make this conditioned in any way.
Of course there are better and worse versions of this that can exist but ultimately I think it should be opposed as a state violation of human rights.