Bitcoins will be always at the top since it was the first cryptocurrency that has been created for people to use and all the altcoins are just created in order for people to have variety of coins to invest.
being first is not the reason why bitcoin is still on top. in history there has been a lot of new things like bitcoin that started as the first but soon they were put aside and were replaced by better things.
the reason why bitcoin is on top and will remain there is because it is working much better than any alternative. it is because bitcoin is the most decentralized and most secure currency that you can hope for. it is because bitcoin is actually used as a currency in many places. and a lot more reasons like these.
as for altcoins they didn't start like this. in fact in early years we only had a handful of them and each of them had a very interesting idea which they were exploring. they were really alternative cryptocurrencies. but nowadays there are 1700+ altcoins in existence and more than 90% of them are useless shitcoins specially the coins with the biggest market caps.