To Wander; The next gen will still have plenty of room to come along to. You can simply delay their appearance a little if your original gen has longer life spans. You can prevent overcrowding in a number of ways as well, one of which is to set who gets fed in a food shortage. This would allow your most desirable creatures to live on and breed and less desirable creatures fade back into the past. Having more time to orchestrate the gene pool could lead to very customized creatures-which is my kind of fun. Your kind of fun could be something entirely different so I think longer life spans should be an "option" and not a "change".
As for everyone elses reply, thank you! The upcoming changes to allow for a life span slider is AWESOME news to me. That will allow everyone to play the game at a pace and style more of their own choosing.