While other coins CAN be used, they fail to recreate the smooth experience of using Dash. That competitive advantage that Dash has in the speed, ease of use, and fees departments will allow it to become more noticed in that space. Of course other coins will also be used, there's room for more, actually, but Dash should carve out a good chunk of that market because of the customer experience.
At the moment, due to the relatively young age of the crypto market, it is subject to manipulation by large holders. As the market grows, it will become much harder to do that. Usage and adoption will be the driving factors in currency valuation, as they currently are in national currencies. This is a marathon, not a sprint, and current conditions will not always rule the day.
I agree in regard to Dash being better suited to use as a currency than BTC. If we just look at ease of use and fees. However in regard to popularity, of course it has some work to do still. But yeah, BTC isn't an ideal candidate to be used as a currency really.
However BTC isn't stable with its current volume. It's still manipulated like crazy. So why would anyone think DASH will eventually have a larger market than BTC and be more stable? What timeline are you looking at, 2040+?