I can't still see the problem. How many players leveled their 49-50 by playing only Lost Skull? Probably no one. With MMAs you can play them practically without any losses and you would need to play only ~230 of them. Compare this with the new army and BK, where you would need 411 BKs to level from 55 to 56 (you get knights at level 55).
I don't think anyone would really do this, but instead play Alibabas to grind their levels up just like we did BKs and Nords when trying to hit level 50.
Surely the loot from BK is much better than from LS, but after you unlock the archipelago and start mining your own granite and have 21 scouts bringing tons of it every day, the BK's loot will feel like getting 800 marble today. Old adventure loot prices will drop very soon to rock bottom, were the new units prohibited on old adventures or not.
Anyway, this graph will give some perspective how obsolete all old adventures will be in the long run. Grinding them will be a waste of time even if you didn't lose a single unit.