Why should there be an Altcoin-Season again?

Hi,

People argue that after a significant Bitcoin rally, an altcoin season always follows. This pattern is expected to recur.

I understand this argument based on historical data points: Investments in Bitcoin can easily be redirected to altcoins since funds, whether in the form of stablecoins or Bitcoin itself, can be smoothly converted into altcoins through centralized exchanges (CEXes) and decentralized finance (DeFi) platforms.

However, a question arises about the billions of dollars currently flowing into Bitcoin: what becomes of them? I presume these Bitcoins are secured in ETF wallets, and it appears that the USD to buy ETFs is kept outside of the cryptocurrency sphere.

So, what is the rationale behind the pro-altcoin season viewpoint?