I offered you the only sane argument I can think off: Whales which are still stuck within the 2017 crash holding huge amounts of altcoin bags, planning to do the last exist scam within the next Bitcoin rally. As noobs look at "coinmarketcap" while Bitcoin is on mainstream news when it's about to hit ATH as usual, those whales may attempt to confuse the noob by luring them into buying some of those pumping shitcoins, only to dump on them bigly. This may be the last time for bagholders to leave the crazy train formerly known as "cryptocurrency diversification".
Other than this theoretical exit scam pump, I can't think of fundamentals strong enough for altcoins to maintain any sort of positive traction long end, and this includes Ethereum, Litecoin, Monero and the usual suspects. The ball is on your court to argument why should anyone bother having serious long term hold positions on any of that.