Well first and foremost the charts speak for them selves the idea of an altcoin bull is a little bit naive if you ask me because the chances of altcoins pumping generally leaving only bitcoin behind would be quite impossible. If you check, crypto currency markets are based on their equivalent market cap most of the time and bitcoin has the highest market cap therefore its price movement is to an extent felt by the whole crypto market.
One of the biggest altcoin season I've ever experienced is the season that happened recently and seems to be fading away gradually though. Literally every coin on the market pumped alongside bitcoin.
Your statements look a bit confusing, I hardly link the altcoins season with what you side at the your first paragraph.
However, just like you said that we had the biggest altcoins season that is currently fading away, I was thinking of the past altcoins season, precisely in 2021 when we had about 70% of altcoins that created new All Time High records which many of them has not broken in the recent Bitcoin pump to its hight of $108k on December, 2024.
Actually, we had few altcoins that followed the Bitcoin trends momentum recently, coin/token like Solana (Sol), BNB, has broken it's previous All Time High of 2021, Ada also followed the current Btc trends but failed to beat it's last All Time High records. Others that tried but couldn't beat their previous record are, Etheruem, matic, Dogecoin, TRX, Xrp etc.
A full altcoins season should cover, at least 65% of all altcoins in the market simultaneously in my opinion.