If you are going to degrade what I farmed as legendary into epic, and introduce its newer version as legendary, that really sucks from collecting POV. At least, make the legendary as soulkeeper (so I keep it in collection with same "progress" of unit count towards higher tiers), and make the new unit epic (switch their names in your suggestion). Also @ main_gi, we got archbishop and fortress in most recent (last) patch, as highly costed base units :) .

Thinking out loud about high costed units (from base):

1. cost is justified for the power => base must be strong already
2. relative power increase goes up in line with relative cost increase in tiers => adding moveset / abilities, that would increase cost on cheap piece by 1 or 2, is more expensive here, so the cost rises quicker
3. all tiers being desirable, but not becoming 1 man army (esp. +++)

Points 1 and 2 build up for 3 to be very coin-flippy. If ~ 30 (on +++) of your morale as single piece can get easily countered, its "useless". If it can't be effectively countered by not inconsiderable portion of armies / players, it's OP. Also including them in your army makes matches more RNG-dependent on what you get matched against.

I'm certainly not against new units of any kind, but I imagine low- or average-costed units are easier to adjust for potential problems / balance.