you'd have to convince the benevolent holders of MSC to vote somehow or else it'd be a bother and participation would drop off
perhaps have a quorum function as well
The voters would all hold mastercoins and therefore have an incentive to vote to addjust the fees in the overall interest of the mastercoin network to keep all the services competitive,spam free and profitable (create demand for mastercoins). Only rational self interest is required from the voters.
You say that after 5, 10, 20 years having to vote every x interval of time. Too long, you forget about it. Too short and you get sick of it.