There is considerable evidence that the fact that the current witness voting page in Steemit only shows the top 50 witness accounts has meant that those top 50 get an unfair boost - see my post on that, here.

Without solving that problem, mandatory voting would just result in the top 50 getting an even higher headstart on everyone else. So far I have heard zero feedback on this problem from anyone who can authorise it to be fixed in Steemit Inc.

I presented an improved witness voting page, here.

If anyone is interested I also suggested a 'vote decay' idea to ensure that old votes have to be renewed for witnesses - meaning that we can't have large stakeholders using the system, voting for witnesses and then never returning. When they do this, the witnesses have a secure position regardless of their behaviour on the platform. They could make the worst possible decisions when it comes to hard forks and no-one could remove them without deliberately investing a large amount of money.

Mandatory witness voting is not a bad idea provided the witness voting situation is made fair.