“Is” arguments are among the most vociferous and yet inconsequential. What “is” money, etc.
The question was why do Bitcoiners consider Monero to be a poor store of value. My answer remains that as a shitcoin, of which there are and will be countless, Monero lacks and cannot gain first mover advantage against Bitcoin.
Your first mover advantage claim has already been disproven with one major example; DNMs. There are also various other white market examples like Coincards, ShopinBit, SilentLink, etc where that is clearly being shown as untrue.
First mover is helpful, but is not some immutable law and guarantees nothing. Ask Myspace, Blockbuster, Sears, Kodak, cassette tapes, CDs, and countless other defunct technologies and brands about first mover advantage. You stagnate you get left behind. The market doesn't care if you were first and currently the most popular.
