This is a really good point, I hadn't thought of this before. Because of how many other options there are, the "scarcity" of Bitcoin is only real if there is something that is going to prevent other cryptocurrencies from being used, or promote Bitcoin above all others. Given that Bitcoin has been demonstrated to be inferior at scale vs. some of the other options, I don't see a reason to expect that it will continue to be the most prolific in the long term. By extension, there doesn't seem to be an obvious indication that any of them will see a lot of use in the long term.