James Jacobs wrote:
If diversity "taints" a game for someone, then perhaps Pathfinder isn't the game for you.
Two things:
firstly- I don't believe anyone said that diversity taints the game, diversity becoming an issue taints the game. If you want a good example of this take a look at what's happening with the warhammer community right now. Lorewise If Korvosa is an area with little outside influence (I can't honestly remember) then little diversity would make sense within the area as the people living there have lived there for a long period of time.
secondly- so long as the character's race doesn't have any major impact on their personality, there should be no issues for a GM looking to recast some people without rewriting the adventure. Just try to avoid swapping a human with a bugbear and it'll be fine.