Well, following Eaty's comments, I still think it would be better/healthier for the game to halve the pvp rewards and introduce a separate queue than to enforce a playstyle every season. As for reasons other people have mentioned, I do not think a second queue would "split up" the playerbase too much, if anything it will make the current playerbase grow.
There are a few options if you work with two queues without necessarily requiring you to sink the economy by handing out too many pvp coins.
For example, you can split up the queue, but the ladder only follows 1 tier. Players would still get a pvp coin per win whether they play OU or UU, but only one of the metagames would have a ladder with subsequent ladder rewards. In doing this, you also have two options:
1. You add a UU queue for people who want to play a different metagame, but maintain the ladder rewards and ladder tour for OU only. Basically, this would give players the options to play UU if they want to (making it a more "fun" and "relaxed" metagame because you are not pushing for ladder (although you would still get a coin per win) basically unranked but with a reason to play it)
OR 2. You add a UU queue, but the ladder shifts every season from OU to UU. So one season, the ladder and the ladder rewards would be based on the OU queue, whereas the other it would be based on the UU queue. (This can be done in the way you suggested with 3 months OU and 1 month UU or whatever ratio you deem fit.) This would challenge ladder players to compete in a new metagame, while leaving other players the option to gain rating and coins by playing OU if they do not feel like competing in the UU ladder.
Personally, I think both options are better for the game than enforcing a singular metagame. Arguments about the playerbase being too small can be easily proven or disproven by implementing this system for a season and monitoring the activity of the playerbase. Maybe I am optimistic, but I do not think it would lead to dead queues.