I somewhat agree with what you've said. Bitcoin is the only "reliable" coin that you can hold for the long term. However, it doesn't mean that people should stop investing in other projects. A diverse portfolio is always better than hedging your bets on just one coin or asset. I also see potential in SOL, ETH, and other altcoins. Of course, research is crucial and you must not invest blindly.
Nowadays Bitcoin is a King of all cryptocurrency and without a doubt if we put our investment in this coin 100% we will got a profit in the near future cause we all know how it moves in the market even though there's a time that the price will decrease but still it will increase every sometime cause the price is too volatile. Also Bitcoin is already proven by some investors that they got a big profits through thier investment. And not just Bitcoin but many more altcoins will increase too but there's no coins who can overlap Bitcoins.