If there is a bad run of years for the team things can change quickly.. I went on the list in like 2002 I think and although it wouldn't have been as big as today it still seemed pretty long. I got my first offer in 2012 and then a week after accepting see the news van Persie was signing for United😂 probably not a bad indicator of the period the club had in store for those early season ticket years.
They've also got better at trying to make sure the season ticket holders are attending with the minimum games numbers etc so that might have an effect. But right now is the high level of that demand but it can easily change in the football cycle.