And that Genie ...
The ignorance and stupidity of politicians aside, I return to the Genie—who cannot be returned to the bottle. Even if every mainstream encryption app could be compromised somehow (and therefore would no longer be used by anyone, of course), you cannot un-make the mathematical knowledge and algorithmic techniques to execute seriously tough encryption. There is basically nothing to stop any competent programmer knocking up some code, in almost any language, to encrypt data on a device. It's pretty unlikely that even quantum computing will be decrypting today's best encryption if executed properly with sizeable keys (and since the "imminent apocalyptic terrorist attack" bullshit so beloved of imbeciles who watched too much 24 is the usual inane justification, even decrypting the data as quickly as a month later is of little use).
Add good stegnogaphy*¹ to the mix and it is just moronic to think that serious bad actors will be much hampered by prohibiting e2e encryption apps—even if you could.
So why do the security services keep misleading gullible politicians? For they must know (a) that they are lying about backdoors, yet (b) keep on lying anyway.
I can only assume that aside from the usual empire-building budget-tumescing nonsense beloved of such people, this is about sheer laziness. The hard slog of humint, infiltration, shoe-leather intel gathering, hearts-and-minds ops, training and employing enough translators, learning about other regions' cultures and habits, using diplomacy and softpower to get what you want—perhaps it's all too difficult, when you delude yourseelf into believing that the computer can do it all for you? I do wonder what degree of self-delusion goes on in places like Babylon on Thames ... and perhaps more so at places like Langley. Judging by the apparently rotten advice they give to the political nincompoops, perhaps they are neither as realistic nor as practical as we'd imagine.
*¹ Whatcha gonna do? Ban everyone from posting poor quality cat photos? Over 2,000,000,000 (yup, two billion) photos posted every day? Any idea how much hidden messaging you can stuff into even a fraction of that?