That's the reason not to do it, but it will be fine until something tries to poke a hole in it. The overstretch is well within the limits of the material in the absence of an external attack.
I have two 700x32/47 tubes but only one 29er tube. As much as I'd like to say it's a temporary solution, it'll almost certainly spend the rest of its life installed. I guess I could go visit Decathlon or the LBS and see if they have more 29er tubes, but, like, urgh, effort, bro.
That's the reason not to do it, but it will be fine until something tries to poke a hole in it. The overstretch is well within the limits of the material in the absence of an external attack.