I think the main problem is is there's no differing pop costs for units. Both a Laser Lynx and a Thor's Hammer Tiger take 1 away from your cap. Not sure how well upkeep would work; it could be pretty horrible on maps like Pie Chart where you're only guaranteed a 2 bar common mine, and totally negligible on maps like La Corrida where you have so much income you don't know what to do with it, those two being the most popular LoS maps because they're roughly symmetrical. You could consider repairing damaged units to be a form of upkeep, but then again it's way too much of a pain in the ass to use garages to bother with that, and Repair Vehicles and Spiders only repair up to 50% and there is no "auto repair nearby units" feature.