Disney sells enough points at each resort to book at a 98% occupancy rate all year round. Banking and borrowing have to be carefully managed to keep the system in balance, otherwise there could be more points in circulation than there are rooms available to be booked with those points. Disney also has the right to suspend banking or borrowing if the system gets too far out of balance. Given that, perhaps putting the banking deadline 4 months before the end of one's UY is a good thing. Imagine if you could wait until the last month of your UY to bank and they suspended banking just at that time. You would have only a month to use or lose those points. With the current rules, absolute worst case you would have 4 months to use your points.
They have never suspended banking or borrowing in the years I've been a member but it's possible they will do it in the final years of the contracts that expire in January 2042, to force owners of those contracts to use their remaining points well in advance of the end date.
In general it's balancing member benefits against the needs of the system to be consistent and plan. While the current plan is different than years before, it's less confusing for most members.