You are flying now and are probably going through the greatest period in your history, but what happens when you go in a downward cycle again? 15,000 fans (your core support) in a 40k stadium will seem pretty sparse.
Still who'd have thought we'd be getting 25k on a regular basis so good luck...
For a club that's only been in existence for 5 (five) years, they have done really well, although most new clubs obviously have to start at the bottom of the pyramid. See MK 'Dons' for further details.