Right now, the majority of coins have an overwhelming majority of their use in just being investments and a space to store value. I don't see any disadvantage in holding my money in Stellar to let it grow, even if it slowed to 2% a month, that's still better than in a bank and i can spend as much of it as i want whenever i want, vs the banks limits on times and amounts. What makes bitcoin any better in that regard?
Also, it is never going to come down to one single coin that everybody uses. There will still be a multiple that are accepted and often bundled together like a co-op or like how we have a few main social media or video sites without just 1 being a complete monopoly. They all have different purposes and even if 2 or 3 share 99% of the same features, the world is big enough to accept more than just 1 everywhere. The problem you highlight is noteworthy, but exagerrated. Most people are not converting fiat to stellar and then to a purchase. They are converting to crypto and spending crypto, with only a few that eventually go back to fiat for profit because they want to buy things not (yet!) payable with crypto. That gives it value, similar to fiat, but better.