First, being a creature of habit has zero to do with the unpredictability of disabilities. One of the big reasons that
DAS holders say the DAS works for them where paid options don't is because DAS doesn't have a firm return time, so if they are unable to make the return time due to their disability, they can still return later.
Second, there is a set number of paid LL access for each attraction each hour, but there is no set number of DAS. Using the logic as you presented it, Disney would have to either sell LL access on an hour-by-hour basis based on how many DAS return times were issued, or take paid LL away from someone who has it because of the number of DAS issued.
Limiting who gets DAS is the best way to keep the number of people in the LLs at a reasonable