In Civ 5, where the number of units you could field were based on the number of strategic resources you control, I would agree.
Civ V had a mechanism where there was a Unit Cap based on your total population, and if you exceeded it your Production and, I think, your Population Increase dropped by X for every unit you exceeded the Cap (It's been almost two years since I fired up Civ V, so don't quote me on the exact penalties). This seems to me like a better proposition for limiting 'Unit Spam': if too much of your population is slogging around carrying weapons instead of producing goods or raising a family, your Productivity and Population should suffer. This would also allow us to Modify the Penalties by, say hiring mercenaries from City States, Barbarians, or other Civs (something I think the game needs in any case) or having Industrial/Modern Era 'Conscription' Policies/Civics that allow you to raise the Cap by drafting a larger percentage of the population: a real 'Total War' mechanism.
Siege also need to join the game.
Ah, yes, the Siege Elephant in the Room.
Siege Unit are tricky, because up to a point they are almost completely divorced from the regular battlefield, and after that point they are almost indistinguishable from regular units.
Quick and Dirty Guide to Siege Units: Battering Rams, Siege Towers, Catapults, and Bombards have almost
no effect on regular military units/targets. Try dragging a Siege Tower across a battlefield, and you will inconvenience the enemy only because he will be laughing too hard to charge and massacre you for the first few minutes - same with a Battering Ram. Catapults, to my knowledge, were used on battlefields exactly twice, once by Phillip of Macedon and once by his son Alexander, and both times they were the smaller bolt-throwers which are only marginally siege weapons at all. The Roman Carroballista or cheirobalistra were cart-mounted 'catapults' that could be used on the battlefield, but again they were bolt-throwers designed to break up dense enemy formations or pick off their leaders at twice the range of an ordinary bow. Firing a .6 meter iron bolt at a stone wall might eventually chip it away if you have a decade or two to work at it, but it is not a serious 'siege engine'. The larger stone-throwers that could smash walls, like the Bombards that slammed 100-kilomgram stones into masonry, could break walls, but also took anywhere from 1/2 hour to several hours to load, aim and fire. They are effective only against units manning walls or sound asleep.
Basically, up to and including the Bombard, all the game's Siege units should really be Support Units, unable to operate alone or defend themselves in open combat.
And then, with the 'field cannon' ALL the smoothbore, muzzle-loading cannon are manufactured and operate in much the same way, and the only difference between 'siege guns' and 'field artillery/batteries' is the size: firing up to a 12-pound shot, it was a field cannon; firing 18 to 24-pound shot, it was a Siege Gun (the 24-pounder was, in fact, considered the optimum balance of weight of shot versus speed of firing and ability to move the gun). Siege Guns very rarely appeared on the open battlefield (Marlborough and his artillery commander, Brigadier Blood, at Ramillies, Frederick II at Leuthen) but it was remarked on as an aberration in every case.
So, after the Renaissance, the game to maintain its artificial division of all Units into rigid Classes, has to 'manufacture' an artificial Class of Siege Units. This is by no means impossible, we just have to 'enlarge' the minor differences between 'regular' Field Artillery and Heavy Artillery.
My suggestions, (from the background of both military history and 20 years as an artilleryman in the Atomic/Information Eras!)
Siege Units:
(
Battering Ram)
(
Siege Tower)
(
Catapult)
(
Bombard)
- These are in parenthesis because they also have Support Unit Characteristics: they have no effect on Combat Units and can 'stack' with regular Land Combat Units
Heavy Howitzer - which can use the current 'Artillery' graphic, because that actually shows a heavy howitzer of the 1915 - 1918 period.
Heavy Rocket - representing the large and long ranged but area effect truck/track-mounted individual rockets of the late Atomic Era - too inaccurate to do much to fast-moving tactical units, but able to do lots of damage to city structures.
The last two can also be used against Land (or Sea) Combat Units, but that's not their primary function. The Heavy Howitzer should have to 'set up' before firing, but the Heavy Rocket launch vehicles are designed to be able to 'shoot and scoot' so they do not have to Set Up first.
Ranged Units:
Slinger
Archer
Crossbowman
Field Cannon - which really appeared at the end of the 15th Century, so is a Renaissance Era weapon, not Industrial, and about 170 years after the first Bombards (dates: 1311 CE first Bombard, 1480 CE first 'true cannon', both in France)
Field Artillery - Early Atomic Era, perhaps at Tech: Advanced Ballistics (first 'electrical' computers were used to calculate Firing Tables for WWII-era artillery) - Graphic similar to Civ V's 105mm howitzer/Artillery Unit graphic
Rocket Artillery - the current Civ VI graphic shows a Multiple Launch Rocket System (MLRS) which, like the Soviet 'Guards Mortars' multiple rocket launchers, is designed for maximum fragmentation-anti personnel effect on a battlefield, not the maximum Blast effect needed to take down city structures.
- And in case you wondered, the Machine Gun, AT Crew and Modern AT units are ridiculous manifestations of historical Ignorance that have no place in a game the scale of Civ VI: all of them represent weapons that were added to other units as support weapons, and should be Promotions or, perhaps in a new category of unit modifications, Augmentations of existing Melee or Mounted units. The Modern Era and later units to make any sense have to be representing Division-sized historical units, and no one no where ever formed Machine Gun, Antitank Gun, or Antitank Rocket Divisions: The Soviet Army in WWII formed Antitank Artillery Brigades, but that is as close as anybody ever came, and those brigades were by doctrine to be used to
support rifle or armored units, not as separate organizations.