The thing about Kelvans in the Milky Way is, Rojan and his friends apparently had adopted the horribly uncomfortable human form some time before
Kirk showed up - Rojan complained how they had to suffer in these bodies on the planet already:
Rojan: "I do not think we could have kept our sanity living too long on this accursed planet."
Hanar: "It is an undisciplined environment. One cannot control it. Yet there are things of interest."
Rojan: "Yes, but disturbing. These shells in which we've encased ourselves, they have such heightened senses. To feel. To hear. To smell. How do humans manage to exist in these fragile cases?"
Hanar: "Since the ship was designed to sustain these forms, we have little choice."
Rojan: "At least we'll be away from all of this openness. This is too strange for us, Hanar. We are creatures of outer space, and soon we will be safe in the comforting closeness of walls."
Sure, the bodies are necessary for surviving inside the human starship - but they seem to have been adopted before the ship arrived, and are the reason the Kelvans suffered on the planet. And getting into and out of the bodies appears to be a complicated process that cannot quickly or easily, if at all, be reversed, giving even more credence to the idea that the new bodies were adopted long before Kirk showed up.
Basically, then, we're being told that the Kelvans can't conquer the Milky Way without ceasing to be pure Kelvans, which will probably mean they will tear their society apart in the process. Real Kelvans will want to, will have
to life in the empty space between the planets. But apparently the planets will still have to be exploited (or the Kelvans would simply have moved into empty space rather than into Milky Way), and that calls for impure Kelvans and the associated suffering...
Rojan reporting back on this unfavorable conditions might actually discourage the Kelvans from coming here!
Although how Milky Way would differ from Andromeda in this respect is unknown. The only known difference between these two galaxies (before Andromeda began exploding, that is) is that Andromeda doesn't have an energy barrier, since this surprised the Kelvans.