I have read several teachers say Aikido is 60 percent ground restraints - and I have felt my own practice move away from that. When I was focused on Daito Ryu EVERYTHING ended in restraint. When I was training with Waddell and Fowler in the 1990s 70-80 percent of technique ended in restraint. One of the guidelines Clark Sensei had was to try to restrain while standing so you can deal with the next guy when he comes in. The only techniques I recall going to the knees was shiho nage and ude gaeshi. As I recall, Clark said in a seminar back then he never really liked those restraints.
Even though I have been a little lax about restraints, we continue to keep the engagement dangerous even when uke is on the ground. He has the right to sweep, pull down and trip tori if he can. This rule has helped us maintain zanshin, good post-waza maai, and it helps lead into more dimensions of Aiki problems and solutions. In Matl Sensei's Judo he insists we reach a place of strength and safety within one step after the throw. He does not emphasize the locks in class, but says if you really need it step on their face. Works for me.
I say there is a curse in much of Aiki. It is the notion that Uke * dies upon impact *and fails to be dangerous while on the ground.
Perhaps we should start randori with one person on ground and one person standing. Alternate positions and who initiates the attacks. I betcha you would get some interesting results.
Another thing to consider - when Daito Ryu was my focus, the restraint typically happened in the moment between throw execution and uke hitting the floor. By the time uke finished falling he was typically already 90 percent restrained.