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.
Bitcoin is not only currently but forever the king of cryptocurrencies. Everyone can make long-term investments on Bitcoin with complete confidence that they will get profits for sure. Because no one trusts any other coin in the crypto world, everyone trusts Bitcoin without hesitation. Bitcoin, on the other hand, cannot be controlled by anyone, it is fully decentralized and therefore can be invested in blindly. Because of these huge benefits, you should invest only in Bitcoin even if you don't invest in other coins