Purism has at least 5k of preorders, so it isn’t a question of waiting for enough orders. The issue is getting the phone to a good enough state that it can be shipped. Nobody is expecting Evergreen to be a polished product, but Purism needs to be sure that the hardware at least is good, so all the rest the problems can be fixed with future software updates.
It shouldn’t cost you anything to get the future software updates (except the time that you spend downloading them). Software updates can make a huge difference in performance and battery life. For example, upgrading from GTK 3 to GTK 4 will bring hardware acceleration to the interface, which will make everything smoother and more responsive (and also more energy efficient).