At the risk of cluttering the boards even more with the fill/secondary support issue...
Everyone understands how the system works currently. That's not how people upset with the system want it to work. Can we all just move past this single point of friction in the debate?
-Low queue times and diversity are not mutually exclusive traits desired of the fill role. -Secondary support players are likely players who actively enjoy the role more than any third option, but don't want it every game. -Human decision making is hugely influenced by convenience. Even if you can manually game the system to grant you more diversity, the lack of convenience naturally drives players away from the fill/secondary support choices.
The support role may be unattractive to the majority of players, but you can't grow the support role by forcing players to participate in it. That doesn't seem to be helping queue times for non-support players, and it will only get worse as the support pool is distilled to support mains only.
The system as it stands is not working for anyone except support mains like myself who enjoy half-second queues. It's not about blaming anyone for why this has come to be. Long-term work continues to be done to make the support role more attractive, but there is no obvious guaranteed solution for making the role more popular. It's probably half as much a gameplay issue as a social one, to start.
In the short-term the system needs to be tuned better to accommodate the needs of players. The other way around should not and cannot be expected.