That's not what the CMB is. The CMB is an incredibly uniform pattern which appears to have been formed by a very hot plasma at some point in the past. It's difficult to imagine an explanation other than the Big Bang (or some kind of expansion of a very hot plasma) because the uniformity means it was almost certainly concentrated in space.

But yes, in the distant future, it will not be possible to detect the CMB anymore (and it's entirely possible that alien civilisations that evolve at that time would probably never have a model of the Big Bang because there would be no remaining evidence of it).