Nothing will change, except by regulations which might make the whole process more difficult, but it has nothing to do with bitcoin being more or less expensive, anyway. You can access a plethora of exchanges and acquire small amount of btc like 1$, depending the platform. Of course you are going to get only few satoshis with that purchase, once bitcoin skyrockets to superior price levels, but that will still be possible, because exchanges don't want to lose their customers and especially potential adopters who start buying bitcoin in small chunks as experiment.
I think things will be easier, once bitcoin become popular and widely accepted, there will be many online payment apps that add bitcoins to their apps and normal users can buy like any other goods. Buying bitcoins on exchanges is simple for us, but for ordinary users, I think it will be very difficult for them, not everyone can do it. But one thing that we will lose when it is widely adopted is privacy, it will be very difficult if someone prioritizes their privacy, hoping the DEX will last until then.
