I think that's a good point, it may be worth revisiting in a few years time when the install base is higher and access to 4k is cheaper. I suspect the next generation of consoles will dictate how popular 4K becomes.
Plus so far every single Ultra HD Blu-ray title has been a UHDBD + BD + Digital Download, which does help customers if they still want access to a BD version or digital download. Of course AL don't have to go through the hassle of a digital download.