Mana Leak in something like Faeries wasn't fair when it was around. The issue about effectively giving blue too many effectively 'non situational' 2cmc counter, the blue based decks just become inherently unbalanced.
Remember Timespiral and Teachings Control. That format didn't have 20 2cmc but did prove that having 8 effectively 'dead' counters in the form of runesnag and mana leak was actually pretty close to enough for blue based control to dominate.
I honestly believe having 'situational' 2cmc counters in the core with the only truelly 'dead' counter being in the expansions allows for a much more balanced and enhanced format.
Regarding the 4 sets thing. We've effectively had 4 sets a year now for something like 5 or 6 years, so how can it be a new thing Wizards or the 'evil' Hasbro has introduced 'now' to milk us players? Magic over the last year or so has seen a dramatic decrease in the number of cards a player needs to possess compared to previous years.
Another thing, about the coreset being 50% new cards. I reckon that was pretty much a once off with many of the cards being simple renaming and 'fixers' of staples from the previous coresets. Its probably going to be more in the region of 25% to 30% new with the remaining cards forming the core 'reprints'.