The funds for witnesses are separate from funds for posts & curation. So even if we reduced rewards for witnesses those funds could not be easily allocated elsewhere.

We are doing hardforks every week. It doesn't take much coding to change the hardcoded proportion of relative witness payout and content creation incentive fund. And even without reallocating explicitely the funds, the simple fact of reducing witness salary will reduce Steem Power inflation and give everyone else proportionally more stake in the system

If their salaries where closely related to their costs, it would be easy to bribe them.

Looking at how much skin in the game a witness has can be a criterion for voters to consider, but that doesn't mean witnesses need to be showered with cash just to make sure they have skin in the game. The mere fact someone can get enough clout to become a witness implies in a large majority of cases that she is either a whale, an Bitshares VIP, or a popular content creators who will have already accrued quite a bit of skin in the game. Beside many witnesses are powering down from an already huge stash of SP so that pretty much everything they earn as witness is just liquidated right away.

Witnesses are right now literally controlled by Steemit. They will obey slavishly to anything Steemit ask them to do because if they don't and they lose the support of Steemit, there goes their beautiful witness position and its generous pay.