I would be more worried about the lack of depth in the gene pool if importations were to cease entirely, Garden, as opposed to a lack of new morphs being created. My opinion only and I don't know that science would back me up, but with the animals currently available, there are untold numbers of morphs still to be created/discovered just from what is currently available. I think the wild imports just short-cut the process by a few hundred thousand years by introducing new genes as opposed to waiting for nature to take it's course and new morphs to emerge from the crossing of the animals that are currently available.
Interesting topic to raise and I'd be interested to see what other people think about that, as your argument makes sense.