as far as I understand it it's based on "current demand", say if an account started spamming comments right now, the RC cost of all comments goes up rather than for that specific account, which obviously has its downsides.

Witnesses basically just decide how much storage fits into a block which got increased at one point when splinterlands showed demand as far as I remember.

This of course reads as a few abusers being able to make comments cost more for everyone which has an attack vector, but I guess we just haven't gotten to a point where it's deemed important enough to think about how we can improve it.

PS. I may not be 100% correct about everything regarding this.