10+ hours a week is not a lot. 8/9 hours a week is considered a light schedule. 12-16 is the standard amount. Also what on earth makes you think it would go down during summer? People raid as a leisure activity, they do it night when they are not at work or school, If they were no longer going to school or work during the day why would they no longer raid at night? Being a leisure activity you'd expect it would probably go up during a time when people have the most amount of time for leisure. Except it doesn't increase because guilds decided on schedules and very rarely change them.
People aren't constrained to full raiding schedules because Blizzard has a gun to their head, It's because they want to do it. Also the fact that It's not new anymore (Which It sort of is, BRF has been only been out for 10 weeks, when you consider the average raid lasts for 25-30 weeks, yes It's still quite new) doesn't mean anything, people raid to complete the raid, If they're not finished and can't clear 10/10 in less time they're going to continue raiding at full schedule.