It all depends on your ability to take risks. The safest investment for a long term is bitcoin or ethereum, of course. Altcoins are more risky, but you can expect a big profit by investing in this part of the market.
It depends on how long to invest money. If for a year or two, then I would agree. If for a longer period, then I'm not sure that Bitcoin and Ethereum will be still on top. Many competitors have come to this market.
In the long run, I still think that BTC and ETH will always be at the top. The altcoin is growing fast, but it can't easily overtake BTC and ETH. BTC and ETH are the two cryptocurrency decisions that determine this entire market. And if one of them is replaced, I think the market will be very chaotic