Owner of the stake votes for witnesses or sets a proxy. Delegatees of the stake don't influence witness voting for the delegated stake. It's still in the owner's power to vote for witnesses with the entire stake, including the one delegated away.

Also by splitting a large account into several smaller ones, you reduce the influence of each account, but you can vote for more witnesses, in the scenario proposed here. The question is: would a whale be willing to do that? There is a higher chance his or her influence won't be decisive if it's spitted.