I doubt any GM in their right mind thinks like that. If Gunnar had come up and hit like Cal, there's very little chance that Elias would be like, "Oh I have to promote player A, B and C also because they'll probably hit like Eddie". It's just as likely that he's thinking, "Oh Gunnar has come on slow, I can't promote anyone else because they'll adjust slowly as well."
Now the overall point of not filling the entire roster with rookies is probably on point. I just seriously doubt that Gunnar's success/lack of success has anything to do with that, which has been pointed out in this thread.