Yup, if I want to level up my units, I'll research upgrades for my units. That requires resources, not combat. I honestly never saw the point of a mechanic or medic in C&C, because if I won a battle, I would usually go on to destroy their infra, not heal up to go again.

Starcraft got this right.

One thing that may be interesting here is a "drill" feature, where you lock up a unit for some time and expend some resources to get a higher tier unit. That way you can spam units early on and replace them later for cheaper, but the total cost would be higher than building that unit initially.