My understanding is Disney rents the rooms for cash so they still get occupied. Isn't this the same as Disney using those points therefore the points are still in the system and did not disappear.
Breakage isn't "Points did not get used." Breakage is "Rooms did not get booked." The short version: any unbooked room at 60 days is fair game for Disney to rent out.
In a perfectly balanced year (points available == points allocated), there is a direct relationship between them: each point not used to book something results in some part of a room-night not reserved and eligible for Disney to rent. But, it's likely that each year is
slightly unbalanced due to banking and borrowing. In a year when many more points were banked/borrowed in than borrowed/banked out, there will be more points chasing the same number of rooms. And, vice versa.
This is further complicated by the fact that there is a lockoff premium (or, if you will, penalty). At resorts with lock-offs, the components of the lockoff booked separately require more points than they do if booked together. Disney sells (and charges dues based on) the resort as if the lockoffs are never split. Each time a lockoff unit is split and booked separately, it creates slack in the system that generates breakage inventory by design.
I suspect most of us would be shocked at how much breakage is created this way.
So, in a "surplus" year, there are two ways that excess points can be absorbed. The first is through the lockoff premium: rather than have all of those "generated" nights go unbooked, some of them will be booked with the excess points. The second is through expired points: a Member who allows points to expire rather than using them makes room for some
other Member to book that "same" unit. As long as that happens prior to 60 days, the expired points do not result in breakage. In a normal year, the "expiration" component of breakage is probably more than a few percent, because even in resorts without any lockoffs (e.g. Poly) the 2.5% breakage cap is hit every year.