> you have total control over the seed determining the next set of valdators

There are many possible algorithms for determining this seed, some of which do solve the stake grinding problem. I personally like the NXT algo; its basic approach in simplified form is:

seed(genesis) = 0

seed(block) = hash(seed(block.parent) + block.proposer.address)

No grind capabilities at all. No matter how many times you try, you will always generate the exact same result. The randomness basically comes from absentee proposers, so the only way to influence the outcome (not proposing) is costly as you sacrifice your reward and all other influence over the block beyond signing / not signing it.