A new signee can get into the top 10 and just stay in the team. Any team could theoretically still have all top 10 riders in the world by signing non-top-10 and getting them into the top 10 (but not anymore by buying them).
If a rider is 10th and there is another rider in the team in the top 10, that rider is free to negotiate with other teams (not already having a top 10 rider). If that other team already have a top 10 rider, they can only sign non-top-10 riders, that are out of contract. If the rider is just within the top 10, he either has a contract (and can just stay) or he has no contract and can sign for a longer period, or he isn't wanted anymore, and that ofcourse limits his options (only teams without top 10 riders can sign him, in theory that's at minimum half of the protour teams, still a lot of options it seems, but ofcourse not really nice if you are just top 10 and your only options are to go to a team with no top 10 rider so you have to carry all the weight.
Again, just ideas.