It's going to be Victoria Justice in 2042. You heard it from me first!
All "joking" aside, I'd be surprised, probably shocked if Drew called it quits within 5 years. It's still too soon to bet that he'll host another 20 years, but I feel like another 10 years is reasonable enough.
More important, I think (I'm with pretty much everyone else in saying that) it's too soon to speculate a replacement unless Carey was miserable and/or got bored. The guys has come a long way & really is in the best shape of his life IMO. I didn't expect it 15 years ago, but as mentioned in chat not too long ago, there isn't anyone on Planet Earth that can do a better job than Drew Carey as TPIR host today.