Why not? there are too many good altcoins out there and I believe that at least 10% of them have a good future. 1% of them can even beat and replace bitcoin. Mark my words. Look at ethereum and litecoin, as well as lumens. Their market caps are extremely big right now
There are a lot of good coins out there but not one that can surpass bitcoin and if that coin does not appear soon then I think bitcoin is going to remain the top cryptocurrency for a long time because the advantage bitcoin has by being the first crypto ever is getting larger by the day.
Nothing can replace the bitcoin position, because the parent bitcoin of all altcoin. and altcoin is the only alternative to get bitcoin. there are many alternative currencies that exist today and there are only a few altcoins that are popularly used. but the future of altcoin is dependent on its parent.