That's one component of the evaluation, to be sure, but the thing I think is underappreciated is the relative cost of expansion as compared to your operating margin as you move up or down the levels. When in IV or V, it's much easier to both field a successful team and still have money left over for investing in infrastructure such as arena or scouting. The higher up you go, the harder it is to have much extra money left over and still be successful. The strategy you're proposing, aiming to expand after you go up, is perilous, as not only will you need to increase your payroll significantly in most cases to remain competitive, but you'd also have to find a way to expand the arena and to do all of this with a lower revenue base than your league competitors.
The way I've always thought about it is that since the seats are a fixed, one-time cost, it's easier to go ahead and get them earlier than later, within reason (i.e., there's no point to build up to a 20k arena while in IV).
Great points! Also, you get a promotion bonus when you promote, so it's easier to sell out seats and charge the highest prices possible following a promotion...so if you wait to add these seats until after your promotion, you are leaving money sitting on the table.
Also, As hrudey pointed out, it only becomes increasingly difficult to do things like Arena projects the higher you go in the game. I always tell people, it will never get easier than your first few seasons in the game. At this point (for me) there is simply not enough money to go around to field a competitive team, and continue building my arena. I was on a very fast pace early in my career building my arena, but my team started overachieving, and I promoted too fast, and found myself spending money on my team to keep that going, and now it's darn near impossible to free up any kind money to finish off my arena without hurting my performance on the court.