Of course. But in practice here's what's going to happen. The good apps, say Postgres, will implement the ability to do this. They will create caches, and release them upon request from the OS. This will significantly hinder the performance of Postgres because it will keep losing its caches. Note that this is worse than just emptying the cache, you actually lose the allocation and have to start over. In some cases you'll effectively disable the cache, which is there for a reason!
Now, here comes, say, MongoDB, which says "Yeah, I have caches, and I need them all! I won't release any allocations because I have to have them." Let's say, for whatever reason you run both Postgres and MongoDB on the same box and it's running out of RAM. Now you are punishing Postgres, the good citizen, and rewarding MongoDB, the bad citizen, and only because Postgres bothered to implement the ability to give up its caches.
I cannot find the reference to this, but I believe this was tried in early Android, and the consequences were that it became unusable as soon as you installed at least one memory hog app that never gave up any memory.