The limit used to be 30. It was increased to 40, then 50, because the queue was always full. Thanks (but no thanks) to the new injection of serial approvers, the queue is now emptied very quickly, so there is no need to have such a high limit.
A lower limit reduces the damage any one person can do, which isn't much help since it also reduces the amount any one person can genuinely help.
A lower limit increases the time it takes for serial approvers to get their coveted badge. Maybe they'll get bored? I hope so, but doubt it — I have seen people suggest exactly 500 (mostly bad) edits in quick succession to get the Copy Editor badge, so there will always be people who are patient enough to game the system.
All in all, I think lowering the number of review votes per day is reasonable: they should be adjusted with the number of edits to review. I expect the limit will be raised again if the queue is full. I myself ran into the limit for the first time yesterday; before that I don't think I'd made 20 reviews in any one day since the review rehaul caused the queue not to be almost always nearly full.