The PIE reconstruction is a monumental intellectual achievement, and yet it isn’t “a language” that could be ascribed to any single speech community at any time. It’s a large set of coalescent reconstructions distributed in time and possibly in space as well. Other protolanguages, even relatively uncontroversial ones, are usually still more nebulous. If we ever manage to prove that the IE languages are related to some other established family, the reconstructed features of the common ancestor will naturally be even harder to constrain, and the protolanguage itself more elusive and fragmentary. It is hard to predict how far back in time our best reconstructive methods can take us before the notion od “protolanguage” becomes too vague to be meaningful. We can only resolve this question empirically, by putting our methods to extreme tests. If we consistently fail, it may mean that we have already reached the limit.
But if you’re interested in this stuff at all, you’ll want to read the whole thing. (That, by the way, is one of the main reasons I bailed out of historical linguistics; the uncertainty became too much for me.)