Why evolution might not be able to allow them to eventually be able to develop permanent adaptation to those environments?
Evolution is good at solving problems, but vastly less so without natural selection processes (competition, reproduction, etc) occurring. These don't happen while tardigrades are inert, and they go inert well before reaching conditions of vacuum or near absolute zero temperatures. So it is very difficult to imagine how evolution would produce creatures capable of this.
There's another and even harsher problem, which is set by fundamental physics. Consider a similar question: "Given enough time, could evolution allow life to survive and thrive inside of stars?". Almost certainly no, because the participants of chemical reactions would be ionized. A related problem exists for low temperatures -- reactions tend to become thermodynamically unfavorable (which is why tardigrades shut down in extreme cold in the first place). We can get pretty technical in examining this too, with things like Gibb's free energy. There seems to be a range of conditions, though with imprecisely defined bounds, where life as we know it is possible, and these bounds are set not so much by evolution, but by thermodynamics and chemistry.
People bring this up a lot but never mention that most of these extremophiles evolved from species that were in a lot more environmentally stable environments or environments more conducive to the production of DNA/RNA like chemical bases.
I'm not sure how safe it is to say that, either. We do not yet know when or even where life first got started, let alone the conditions present. We think it is pretty likely that RNA developed before DNA, but we do not know all the details for how RNA arose, or how far down the road of abiogensis it was, or the boundary conditions necessary.
A further complication is that there may even be multiple shadow biospheres, with independent geneses, that we haven't yet discovered (a great deal of current research is pursuing this).