Abuse of the rental system aimed at getting a wanted color is a possibility, but I’m sure devs have something in mind to mitigate that.
First of all, is it even technically viable to get limitless rentals? 100 worlds attached to one public world - all visible in the sky?
There must be some idea behind the rental system to stop any kind of abuse. Simpliest would be to put limit on number of worlds per account. Another, to apply some sophisticated price scheme discouraging any manipulative behaviors (like dropping unwanted world and renting another).
I suppose it could work this way as well:
FIRST OF ALL - rent only one world at once. You cannot apply for 2, 3 or 10 worlds together. Just one. SECONDLY - you may apply for more additional worlds after you finalize the first one and after one full month has passed (and only if you keep the first one online, so if you pay beyond the first month)
SO, if you decide to stop paying for your fist world, the new one you get needs to be online for at least 2 months before you can rent another one (so you will have to pay for 2 months ahead). If you discard the second world after those 2 months, the new one you get then (and your third) will have to be online for 3 months (you must pay for 3 months ahead) before you can apply for yet another one (and your fourth).
Long story short: If you don’t discard any of your worlds, you can get a new one every month. After a full year you will have had 12 sovereign worlds to your account, providing you kept paying for all of them. Every time you stop paying for one of your worlds, the waiting time for the new one is increased by 1 month.
Also, for every 3 worlds you discard (stop paying for), one of your locked worlds (the oldest one) is permanently deleted. There can also be an additional renting ban for every 3 discontinued worlds you had (let’s say renting a new world unavailable for 6 months, and that would be on top of whatever delay you “earned” by discarding worlds earlier).
These are just rough ideas and numbers, don’t argue the actual details, please. Just think about a possible way of managing the rental system so players can’t run a rental roulette in hope of getting colors they want. Slowing the renting process seems the most obvious to me. It doesn’t place limits on actual number of worlds you can have, but it does make the process of acquiring them pretty slow. It’s some kind of reputation-based system. Constant discarding freshly rented worlds is punished with delaying of getting new ones.