You ever see the South Park episode where all the men in South Park stick their balls in the microwave so they can get testicular cancer so they can get weed prescriptions? Well in the same episode they also ban KFC from South Park. One of the doctors at the end of the episode concludes that the reason why all the men were getting testicular cancer was because KFC was banned from South Park, that KFC was keeping people healthy. The point being that correlation does not imply causation.
Just because a fill player in season 6 gets shorter queue times in season 6 for selecting fill does not mean that they select fill BECAUSE they want shorter queue times.
Most of these people were fill players BEFORE season 6, when queue times were not affected by role selection. Therefore, those people are obviously not selecting fill for shorter queue times.
These fill players pick fill because its an easy way to not have to play the same role over and over. They don't care what role they play, they just don't want to play the same role over and over. With the new system in season 6 if you select fill you are given support 90+% of the time. So basically this new system has taken an option that players originally associated with "not having to play the same role over and over" and now made it so that if you choose fill you play the same role over and over.
Can you understand why fill players are frustrated with the new system now? Our whole reason for selecting fill has been taken away which has caused many of use to stop selecting fill. Since us fill players used to play support 20-30% of the time and now we aren't playing support anymore because we don't select fill anymore that puts a huge strain on the matchmaking system and increases queue times.
If Riot wants to increase its support player base and decrease queue times the easiest way to do it is make fill a truly random option (i.e. equal chance to get all roles.) This would make fill players start selecting fill again which would increase the number of players willing to play support.