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.
Point is that you dont have to vote. Only if you want change. fees stay the same if no votes are cast. lazy smart users could create an auto vote wallet.
The good thing about this system is that we only get the people for whom it actually matters voting consistently. people who don't care don't vote.