if you're gonna shoot down the idea, at least give an explanation of why. Specifically answer this question...
How does fleet upkeep remaining at high levels when fleet supply isn't used make the game better than if it were to regress to a lower level?
I'm not the guy who shot you down, and to be honest I don't have strong feelings either way on this, but in an attempt to answer your question....
I think the logic is that "fleeting up" should be a strategic decision, and it shouldn't be a decision taken lightly. You shouldn't flippantly decide to "fleet up." Rather, the question you should ask yourself is, "are you SURE you want to do this?" I find myself quite often waiting to "fleet up" for this very reason. The longer I wait to do it, the more money I save, and the more eco I generate. However, it is a risk, because I can be attacked by an opponent who has "fleeted up." Similarly, he takes a different kind of risk when he fleets up. It's a trade off.
A compromise between what you want, and the system which currently exists, would be to pay money to "fleet down."
Personally, I've always preferred the idea of fleet scaling with the number of planets owned (you'd get a certain number of fleet points for each planet).
Having said all of that, I have no particular beef with the system in place.