If I'm not missing something, folks are conflating two different senses of "proto-language" here.
Proto-language, in our conlang parlance, means "an older stage of the 'final' language I want to create". In the historical linguistics sense, it means "reconstructed common ancestor of two or more attested languages". Therefore, e.g. Sanskrit or Classical Latin, ancient as they are, are not proto-languages in the latter sense. While according to the former sense, Old English would be a "proto-language of English", or Old Japanese would be a "proto-language of Japanese".
If you're interested in doing a diachronic a posteriori language, there is no particular reason to start from a reconstructed example. If anything, that is going to greatly limit the amount of information at your disposal. You can well pick an attested language as your protolang and proceed from there. If you don't care about putting the language in a plausible modern-day setting (a "lostlang"), you could even start from a modern, well-described language, zap any vocabulary like "computer" sort out of it, and pretend to evolve it as if it were spoken thousands of years ago. Like start from Modern Turkish and apply a bunch of sound changes inspired by the development from Proto-Oceanic to Māori?
(Incidentally, the Proto-Polynesian / Proto-Oceanic / Proto-Malayo-Polynesian / Proto-Austronesian series is quite well reconstructed as well, IIUC.)
Another possible point against using reconstructed bottom-level protolangs is that a believable development of several millennia is goign to require working out half a dozen intermediate stages, a shifting palette of loanword influences, etc. But if the scenario is something like suddendly dropping Old Tupi into 17th century Korea, it's entirely OK to enforce some drastic sound and grammar changes that, at a more "natural" environment, would have taken a millennium.
Sanskrit might be one of the best compromises between ancientness, well-describedness, and sources-being-in-English-ness, actually. Yet I don't think I've ever seen Indic bogolangs?