Depends where the metal costs are compared to now.
If the total metal income does not change and the maintenance is flat-out taken from it... yes, but it will cause issues. So, this would have to come with rebalancing... Example: if "current metal income" is "100%", balanced could be changed so:
- full fleet cost = 75%.
- no fleet cost = 250%.
Overall refleeting is faster with those numbers. Of course, if you go this way:
- full fleet cost = 25%.
- no fleet cost = 100%.
This last way won't work. Of course, simply adding metal could also work on its own. Maintenance costs have the "advantage" of limiting income when there is a full fleet, which I generally see as a good thing (from a balance point of view). That said I'm neutral toward having metal as maintenance in the game, or not.