What it will take for me to purchase Dom III

[Agrajag]

I think the problem lies in the fact that summons get stronger as the game progresses, while regular national troops do not.
If it was possible to divert magical research to technological research to improve your troops, it might contribute towards making national troops viable later in the game.
Maybe you can add a "techonology" tab to research, where you can choose to spend RP on technological advances, so in that tab, instead of "Enchantment" and "Conjuration" you would have "Infantry Weapons" and "Cavalry Shields", and instead of spells you would have upgrades, such as "Sharpening Stone" which increases damage for all weapons by 1 or "Combat Training" which gives all newly produced units an expirience star etc...