It's very uncertain for an Altcoin to overcome Bitcoin.
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.
There are many altcoins with their names as Bitcoin forks, but they died with time too.
How many Bitcoin forks are there?Altcoin blockchains are worse than Bitcoin blockchain in security too.
https://howmanyconfs.com/People if are smart enough, will never want to exchange the best (Bitcoin) to worst ones (altcoins).
Why is it impossible? I think that's not impossible because first of all, nothing is impossible and second of all, anything can happen that will change the fate of cryptocurrencies and reshape the current image.
If Bitcoin doesn't change, it will not be a good payment method and in terms of Bitcoin payment adoption, it will fail and here is the room for other cryptocurrencies to shine but at the same time there are other benefits on the table that no other currency has and that's decentralization to my mind. At the moment, Bitcoin is probably the most decentralized coin.
By the way, Monero is good and I think it's not only a close thing that comes to Bitcoin, it's better than Bitcoin in some aspects but it's disadvantage is it's biggest advantage, ANONIMITY. Monero won't be widely adopted because of it's very privacy-oriented nature.