When I’ve seen tachyon’s mentioned, I think they’ve always been concieved of as a sort of “toy particle” that some models could be construed as allowing for but that no empirical evidence suggests the existence of.
What I’m asking is, are there theoretical considerations (any theory/model/whatever that is taken seriously by someone other physicists take seriously) which go so far as to imply that tachyons do exist?
Really, the only reason anyone thinks that tachyons might exist is that nobody’s come up with any reason why they wouldn’t. But then, even if they do exist, all of the known self-consistent theories which allow for them also predict that they’d be absolutely utterly incapable of interacting with tardyons in any way. So it doesn’t really matter whether they exist at all, and depending on one’s philosophy of physics, one might even say that they don’t exist.