You look at tier progression as vertical replacement instead of horizontal balancing.
Like look at Dark Warrior (T1) vs Dark Knight (T3) even if you buff Dark Warriors to the same EHP stats you would still want to bring Dark Knights for their mobility and special ability but at the same time Dark Warriors give player an option to have Shock Units vs heavy Polearm armies.
Horizontal balancing gives players tools to adapt to different situations this leads to unit variety in the late game.
Also monetary cost isn't the only cost that units can or should be balanced around. If we are talking about previous proposal then cost or investment comes in a way of hero skills. So this makes low tiers cheaper but more hero dependent and heroes more skill heavy vs higher tiers that come prepackaged and less hero dependent but this reflects in their price.
You are mixing two different things here. In this thread, as the title says, the problem is "mono-stacking" and, as a supposed reason for that, "enchantment stacking". And OP comes from MP, not SP.
T1 units are not in any way up to debate here. What is are the T2s gaining from Tome of the Horde and Mighty Meek-
Now, T2 - about half of the T2 units, bot cultural and Tomes, are ... Support units. As I've been mentioning a couple thousand times now, Support units are no battlers. They usually have a single magical attack (so they can keep the distance) and supportive abilities. There are virtually NUMEROUS ways to increase their survivability (increasing HPs), but they actually LACK IN SUPPORTIVE CAPABILITY. You don't fix supports (note that OP claims everything is alright with them) by making them T3s from the getgo (you'd still prefer T2s with more fighting nous). Supports simply need better support abilities that make you want USING them.
Then there are the ranged T2s: Fury as a cultural unit which becomes mounted if you pick the trait, Afflictor, which is pretty good due to their great one-shot ability in addition to their regular attack (is in Alchemy T1 Tome), and lastly the houndmaster, which summons an additional unit as help (Tome of the Horde). They are good as well. You can basically use them right away, but keep in mind, if you research these as your second Tome (Zeal first), your third tome may simple make them obsolete (Winds or Glades). I think, as I said, the problem here is that the peak of ranged units is available too early (as early as your third tome). They both should go to a T3, giving the lower tier Archer types more time to shine.
Then we have Battle Mages, acouple of them, Warlock is cultural, then a handful of Tome BMs, White Witch Pyromancer, Lightbringer and Evoker, I think, with Lightbringer being available in a T2 tome. I have no idea how the AI handles them (specifically) in combat, but Lightbringers DO try to convert. Did someone ever try to mass them? We have the Watcher as a T3 which is pretty good, imo, when we talk Battle Mages, then Banshee, plus Transmuter and Chaos Eater as T4s. BMs are debatable, but here I would also say that the problem, if there is any, comes with their magic ability(ies) and their availability, if anything. An early BM like Warlock, Pyro, Witch are not wrong, if you go that way, and the Watcher "upgrade", but I'm not sure the Scrying Tome is really a good pick except for that Watcher - making the T2s better would make this an even weaker pick.
Then we have the fighters (lots of animals, some of which evolve), the Shields, the Polearms, the Shocks and of course the Skirmishers, and it's possibly in this field where you get the most "upgrades" as T3 and T4. But frankly, since there ARE enough upgrades, it doesn't actually matter much whether they come as T3 or T4, does it? It just changes the game a bit.
Lastly - units are meant to die in battles. I know, you try to minimize losses, but there is nothing wrong with a T1 dying in some autocombat and being replaced with a T2 or T3 or T4 or T5. It's the way things go. Lebkuchenhausmeister made a good point which is my point about the 100 units achievement in AoW 3 from a different angle. Being able to play the game with three doom stacks is, what sucks. If you had to mobilize the stuff you had to in AoW 3 things would look a lot different with usefulness.
As it is, you can bolster your T1s with Tome of the Horde (and later with Mighty Meek, plus, of course, with enchantments). You can also leave the T1s more or less right away (not bothering with Tome of the Horde and immediatel upgrade with T2s and after some time with T3.
There is nothing wrong with that and with the T2s, and whether the T2s "survive" midgame depends on what units they are. No matter what you go for, the fighting units probably won't, since there are good upgrades everywhere along the way. For the rest? T2s are ok, with some having "issues", but individually, while the supports are generally not supportive enough for my liking, as their general issue.
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