Attempting to approach the design of the final battle from the perspective that it is a "natural process of competition between the Player and the AI in their ability to play this game" would be a failure. The player is a priori better at using the game's mechanics than the current AI, and will beat it 100-0, all other things being equal. And, assuming that the final battle should be, at the very least, not trivial, and therefore not boring for the player, the odds need to be shifted somewhat in favor of the AI (that is, effectively removing these very "all other things being equal"). This is usually achieved by simply increasing the army and/or stats (and at the same time leaving the traditional balance hole in the form of a "meat-grinding strategy" for the lazy). However, this is boring, unoriginal, and cliché, and no one will be surprised today by someone "killing 69 trillion dragons with 1 skeleton." In order for the map not to be just another product from an assembly line, map designers, including myself, invent new mechanics, as much as the game engine allows. And they are right to do so. And your post contains approximately 0 arguments in favor of changing these trends.
Plus, this is an undeniable exaggeration that has nothing to do with reality. In a battle with a mage, only 4 elemental shards are summoned each turn of the mage (if the shard from the previous turn was destroyed), and demons are summoned once to replace the mage's troops when they die. This is generally insignificant and, in terms of mechanics, is somewhere around the final mission of the original Heroes 5 campaign.
And I don't know who this "interest" (in the finale) is aimed at, but when you play Heroes, and then at the end you get into a mix of Elden Ring, DAO, and God of War, you are slightly puzzled. Of course, maybe I'm just old, and now the younger generation enjoys this, but unfortunately, it doesn't appeal to me. (
No offense - you create excellent scenarios. Don't surprise us so much - we're already interested. Thanks again for the game.