Yes there are plenty of non-donor seats available, there were many open non-donor seats last year, but the donor sections were full.
I suspect they know how many non-donor season ticket holders they are displacing and how many non -donor seats they have available, I can tell you even last year there were a lot of those in the other corner still available, as well as other non-donor areas.
I also know they had more donors in States, Cardinal, and Gold than what they had seats available for. possibly other donor levels as well. But they had plenty extra non-donor seats. So by moving students out of those donor seats to non-donor seats it will give more seats to donors and still allow plenty for non-donors. The vast majority of ticket holders donate at some level.
I suspect the non-donor sections will continue to be reduced as years move on, if things continue on the trend we are on. At some point to get a season ticket you will probably have to donate at least to some level. Maybe that will only be $100 per year, but I can see it coming.