I'm inclined to agree with you that paths can be arbitrarily long as a matter of principle. However in practice I believe PATH_MAX on most distros defaults to 4

I'm inclined to agree with you that paths can be arbitrarily long as a matter of principle. However in practice I believe PATH_MAX on most distros defaults to 4096 so if you're going to insist on a hard upper limit it's at least within reason.