I know we're always trying to keep the game fair for paying & non-paying customers, but I thought that it would be a nice boost if the extra planet/moon field purchases would reset themselves after one year from purchase and allow you to buy them again.

That way you could buy them again after 365 days and raise the number of fields by 5 each year.

Surely this wouldn't create any unfair advantage (it's only 5 extra spaces and a very long wait until the next 5) and I'm certain that sales would increase if players knew that they could keep adding fields each year (instead of wondering whether it's a better idea to destroy the planet and go for more fields this way).

Allowing people to buy the 5 extra fields once per year is hardly a huge advantage for paying players compared to some of the others, like being able to finish ship/defensive construction INSTANTLY with DM , or research for that matter.

Same with commanders and their bonuses.

But this would be a steady revenue stream for the company if lots of players were to add the 5 fields yearly to all of their planets, creating a more steady and long-term use of DM with little direct implication against non-paying players (compared to the currently-allowed instant building of fleets or cannons).