Altcoins are all cryptocurrencies except bitcoin. Among the altcoins, there are both promising, in which you can invest, and useless. Therefore, it is stupid to even raise the question of whether it is possible to invest in altcoins.
One cannot be absolutely sure that Bitcoin will continue to rise in value even in the next few years. Nor can anyone guarantee that in a few years its price will not be even lower than the price of some altcoins. Therefore, you should not bet on only one type of cryptocurrency, no matter how promising it seems. This also applies to Bitcoin.
We must ourselves assess the situation in the cryptocurrency market and not always be guided by the opinion of others.