I don't know. I'm still getting my head around how someone can promote, make playoffs in the first season at that level, then demote the next season and still support the idea of "opportunity cost" as a good thing.
Try again. No one says opportunity cost is a "good" thing. All I've done is try to explain it to some people who obviously don't understand it. Add one more to the list, eh?
The counterpoint is that the opportunity cost of prioritizing spending enough to win your current league is twofold. First, you'll end up in a higher level league without the financial ability to spend as much on weekly salaries as those who invested more into arena building. That's something that eventually evens out. Second, and more important, you lose out on the very valuable experience of learning how to compete and win while spending less than your opposition, which is probably the only way to success if you're not going to accumulate massive amounts of cash.
But the easiest way to look at it is this: if you can't afford to compete at a given level while spending money to improve your arena, how are you going to be able to afford at a higher level with the same arena (which puts you at a further economic disadvantage) when teams that do have more finished arenas can outspend you? The answer to that question is one you'll find through some reflection or perhaps you never will and you'll always be in the cycle of winning here, not being able to compete there.