There are three potential fixes:
1) Separate the queues - a fine solution, on the surface.. but I think it'll create big meta-shifts in what people queue for. Long term, DF parties will have a harder time getting formed or filled in this two-tiered process.
2) Blind entrance - No more 23/24... Players will have no indication of what kind of PT they been paired with until they actually enter the instance. (this would require a 20~40-sec delay to join in-progress groups in order to match wait times for starting groups)
3) Penalty for withdrawing - I'd rather they not go this route...
Regardless of the above approaches, what is most needed is single-click DF acceptance. Payers should not have to re-accept every time someone drops.