B&HA have got several advantages: 250,000 in Brighton and a large surrounding catchment area; no real neighbours or rivals for 40-odd miles around; a relatively affluent population, amongst others.
To think that it's purely success on the pitch, and promotion specifically, that will keep numbers...
No. It's a 9% drop, of about 23,000, which amounts to 23,000 x 0.09 = c2,000.
And this figure relates to renewals. In other words, if the club manages to sell c2,000 season tickets, that'll be the same as this season; if they sell more than that, it'll be a higher number.