Because if another coin even gets close (same order of mag) to bitcoin in terms of market cap, it seriously erodes the "scarcity" feature/argument of bitcoin, which is one of the most critical arguments for putting any wealth/mindshare/time/effort into bitcoin (and by extension, any crypto). Guys like Schiff and Rickards would be proven right, and no one would be comfortable putting any wealth into any crypto for a long long time. Thus, everyone would lose. Humanity would not see the benefits that decentralized money can bring for many years/decades longer than if bitcoin simply becomes the obvious-to-everyone non-dethronable crypto store of value.
To be clear, I think there's niche value in some alts, and some of the experimentation is valuable. But you guys who think that many coins can live side by side with similar monetizations are missing the key point that if that happened, we'd all be sitting side-by-side at *trivial* market-caps, not big ones.
Interesting viewpoint, but I don't believe it to be true. Does silver as a store of wealth erode the value of gold being a store of wealth? What about platinum? There are hundreds of altcoins, some of which are worth tens of millions, some of which are worth nothing. The value of one doesn't affect the value of others to a huge degree. Bitcoin's value is based on the network effect, if another coin gained a huge market share they'd both have value based on their usage and their users. One doesn't degrade the other. Not every coin can have a huge network effect, therefore what you are talking about where there'll be thousands of equally valueless cryptocoins will never happen.