In the past the way it worked was as follows.
Bitcoin pumps, then it seems to stall while altcoins seem to pump and then eventually they both top out.
What happens is the whales pump bitcoin first and while people are piling into bitcoin they get out and move into altcoins and then they pump the altcoins. So later altcoins rally and typically the whales move into stablecoins and then the market peaks.
This might repeat but there is no gaurantee.
Some of us still believe in such a scenario, which is why they buy more altcoins than Bitcoin. An altcoin season is expected to arrive when Bitcoin investors shift to altcoin, but you're right that such a scenario is never guaranteed to occur during this bull season. Of course we can utilize both (bitcoin and altcoin) to get bigger returns, but especially for long term investment then I would still suggest them to hold bitcoin instead of altcoin.
Many of us expect short-term gains from altcoin, while long-term gains from Bitcoin. I own both assets and they've both yielded returns so far, but I'll definitely hold Bitcoin longer for a larger purpose. Altcoins are not for the long term, although I have some of them in my investment portfolio so far.