I agree that btc is good for long term but you cant ignore other projects that are also has good use case and potential. Its just that you need to be very careful cause some are really money scum bag only.
Sifting through to pick out good coins with potential out of a whole bunch is a huge task. There are thousands of altcoins with majority turning out not to be profitable in the long run.
Sticking to BTC is a safer option, to taking a gamble with altcoins. This is my personal opinion.
The comment above yours mentioned forms of diversification like real estate, gold, stocks, which are much preferable to altcoins if done right.
- Jay -