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.
Well it depends on the personal preference of each participant actually. Because if you are here long term like 10 or 20 years of investment, of course the most reliable digital asset that you can invest to that is available right now is Bitcoin. However, if you are just here short term so that you can play around with daily fluctuations then you can trade with major Altcoins like ETH, SOL, VET. Because there's no worry on it years from now. you're just there short term. You don't worry about fundamentals, you are just interested purely in possible 10x 100x or 50x of price of the coin short term.
Bitcoin shows its potential already from the previous years, the ups and downs, news related, crash due to covid and etc. People still keep trusting the process of the bitcoin every time, and we cannot deny this people now keep accumulation after the bitcoin make a halving bitcoin repeats its history, those coins are paired now with the bitcoin so if the market goes down they will follow. Those altcoins have their own uses not only pair of it so still possible there's a manipulation on it than the bitcoin solely itself. Holding is the way if you want to keep doing the lazy process of buy low and sell high that's all no matter what the current price right now. People doing this because they don't have time to become active in market and just use those investment as possible extra income.