I guess the alternative is just bounding the number of instructions and proving inside or outside your language that it is sufficient for your program.
Indeed, that's what bounded means.
Yes, but the point is that you are giving a constructive existence proof for that bound. You asked for why there are so few total languages. The answer, as far as I'm aware, is either brute force on a fixed amount of memory or giving provable bounds either by decision procedure on a restricted language or having to manually give a proof on a more expressive language. Is that that an answer to your (presumably rhetorical) question?