yeah, i don't buy those arguments because you can't replace dedicated developers and a robust, active network of users. so i agree with you there. but i don't think we should write off altcoins entirely. from the perspective of both scalability and governance, it seems impossible that BTC can fulfill all possible use cases for crypto-economic activity. and as satoshi said, bitcoin's core design is set in stone.

even if we believe that bitcoin can port any and all features from altcoins (on what timeline---decades?), its fundamental economic system (POS vs. POW) and consensus mechanism (blockchain vs. hashgraph or DAG) will never change. and i don't think we'll ever see default private transactions on the mainchain, just sidechains, which are essentially altcoins themselves. and with the advent of decentralized exchange, sidechains become less attractive from a bootstrapping perspective.

so while you're correct regarding BTC's use case (and the altcoins that try to replicate it), there is inevitably lots of room for novel, private and experimental altcoins to grow alongside BTC.