When they were discussing the addition of goliaths, they said they aim for two species covering a niche. So orcs and goliaths are the two "big and strong" species with the Powerful Build trait. That way players have a choice when going for a particular theme.
The unstated corollary, then, is that three species covering the same niche in the PHB would be overkill. It's okay to have redundancy in a supplement like MotM that covers a lot of species, but PHB real estate is limited so you have to divide it carefully.
By that logic, I think gnomes and halflings cover our two "small species" slots. If you want goblins to be promoted, one of them has to be demoted to make room. And while I'm not saying it would be the wrong move to remove one of them (gnomes, it's totally gnomes, 4e got that right), there will be people who kick up a fuss at it so you have to be pretty confident in the move.