Agreed in most parts
Dragons are the only thing that costs in CoE (and golems that consumes a mine).
Upkeep free magic units in dom was an early design desicion. I have thought about it several times. I will probably make other desicions regarding upkeep in future projects.
Dominions was designed to be a game system, into which you could fit new nations. We did not intend magic to be as generic as it became, but with the limited amount of national summons etc the game was rather converegent at first. Attempts to alleviate this has come a bit on the way, but it will never be fully accomplished.