Here we are talking about installing PostmarketOS/Linux on a smartphone. The next milestone is not to get everyone on it. First we need a base of early adopters that are willing to use it despite the drawbacks. The more user those alternatives will get, the more they will be developed, the better it will get.
Sure, for the next years, it will be way behind Android or iOS in terms of ease of use, but that's the price to pay to get back control on the device you own that is probably the main computer you use everyday.
For me that's not worse than using Linux in the early 2000s, and like Linux in the early 2000s, it may even be _fun_ to be an early adopter of Linux on the smartphone.
Now we don't need to migrate everyone to PostmarketOS, we _just_ need an alternative OS for at least the ones who are willing to play with it.