What are you talking about? In the previous estate system I /always/ had room for more knights, now there really is a limit to how much knights a realm can accomodate... Which is awkward at times.
Nothing forbids you from making 100 estates with each 1% of the land in each regions. However, what you should really be doing of course is to get more regions, which is the whole point of the new system.
A realm like Summerdale for example would have crumbled long ago in the old system. Now they managed to survive with less nobles than regions, and when new nobles arrived they were able to absorb them and were ready for war. Overall, I think this is a perfect example of how the new system is a success.