I have never really restricted screen time for my sons but their days are interrupted with other stuff so they don't game for 12 hours straight 
They are now 16 and 13 and are at a state school. They chat over headsets to their friends either whilst playing the same game together or just chatting whilst playing different games. Plus they discuss homework.
Ds2 has a weekly D&D meet up online with his friends, they play for 2 hours every Saturday because logistically they live miles apart.
Both my children do a sporting activity, both read books, both are in top sets and have an outstanding effort grade in all subjects. We eat as a family, watch tv show together and play co-op board games together along with poker and gamble for chips. So there is a lot of other stuff going on not just gaming.
There computers are in a room downstairs not their bedrooms. They know homework has to be started the day it is given.
It is a huge part of their social lives a lot of the time because they are chatting as they game. There is a TED talk on the benefits of gaming on YouTube. Ds2 watches lots of physics/engineering stuff on YouTube too.
I think it is about balance. I would much rather they were here gaming, chatting to their mates over headsets than out on the local park getting up to goodness knows what. My friend restricts her son from gaming so all he ever wants to do is leave the house and hang out with his mates on the streets because it is boring at home.