It's not rare but impossible, so far no altcoin succeeds and thousands of altcoins died including Bitcoin forks.
Because Bitcoin is the only cryptocurrency that is decentralized from network to development. The only altcoin comes closely to it is Monero, and you will fail to find other altcoins that can be decentralized.
I'll play devil's advocate and say that you should never disregard a possibility. The chance of this happening is most likely above 1%, which isn't that small when we compare it to probability of you winning a lottery, or even the probability of having an accident, yet people win lotteries and have accidens, right?