Some problems: Collecting small sums of money from lots of people isn't very cost efficient. OS teams don't have the manpower and tools to handle this at scale. There is no penalty for not paying (or enforcing something like that would be too expensive), so millions of users will just trial the software indefinitely without paying. In company environments, small regular payments have a very huge cost-overhead due to the formal processes involved and compliance/regulatory rules.

All this would most likely kill the attractiveness of the solution and one would switch e.g. from Ubuntu to another Linux distro not requiring a perpetual license.