From their 2014 preliminary draft Master plan (PDMP) aviation only provides about 11% of total revenue and that proportion has not changes substantially in the past 10 years.
I don't have a recent annual report, but in 2003 their total revenue was about $10M and (from the PDMP) that has been increasing at about 6% annually.
I too am pissed-off that this discount only applies to new users.
If they lowered their aviation charges to everyone then they would make more money because more of the current users would stay and the nett user-base would increase.
The 2014 PDMP states that they have *plenty* of parking spaces, so their cost to have more people park at YSBK is essentially zero.