This is a good idea and an interesting proposition for bridging two conflicting ideas:

1. that one should have skin in the game (something at risk by ones actions) and,
2. keeping the game theory aspects of up and down voting intact

We can debate the 5x factor specifically, but that down votes should "cost" more than an up vote in terms of recoverable vote power is wise.

I've seen several ideas about this and I will generally side with ideas which are simple and a clear, direct aspect of the system.

For example, I disagree with the suggestion by @kyle.anderson for this reason. He suggested:

A better solution lies around forming a better resolution system where stake can be pooled for the purpose of manually restoring payouts to posts that have been abused.

Not to single it out for any other reason but an example, but his idea requires much more additional code and collusive action. Your proposal allows for the same system of independent action which is collaborative.

The difference between collusion and collaboration are that the former is planned, organized and arbitrated. The later is not necessarily so, and can be the independent alignment of views. It's this kind of stuff that allows for people of disparate views to come together to achieve something together without planning it, and a wonderful feature of the current system which should be preserved. I don't think we want to "bake in" collusion when so much effort has been made to reduce and disincentivize the negative effects of it so far.

As a related side note, the "flag" terminology needs to be removed from the steem front end finally. It only adds to the confusion. Keep the message imparting the seriousness of it, but remove the terminology.