I like that much better.
A combination of activity requirements seems as a good way to get more interaction from less pronounced players and demotivate voter-accounts to participate(or make them a more useful active player).
With voter-accounts i mean... "real or fake players whom sole purpose is to vote on elections by specific orders. They come in this game just on election-days and usually don't do anything else at all."
Yep, it will be a great measure the activity thing.
But although you set activity measures, you still will have in some countries, 10 active political parties at least. So, I think activity requirement measures are complimentary, but not substitutionary of erasing Top 5.
We could say an argument that says, yes to erasing top 5 and adding activity requirements for voters I would vote Yes to that!