SATAN, I believe you're misunderstanding the set up of a defined benefit scheme. Any 'surplus' is just that, a surplus, i.e. not needed. The sum of money that is left over after current and all future liabilities of the scheme are taken into account. The surplus isn't used to pay future benefits. The trustees ensure that the pension fund has enough money to cover current and future liabilities. Therefore, from a financial point of view, the fund being closed to new joiners should have no detrimental effect on the health of the scheme.
But someone please tell me if I've got the wrong end of the stick.