I would suggest that this was set up deliberately for this kind of situation.

The strong players who spend most of their life playing the game and are likely to  be the biggest spenders are not going to be bothered by this,and will switch anything as they seem fit to do.

So its the weaker players who spend the least money,and time playing, who suffer disproprtionally, and so disincentivate them continuing playing,but this is the absolute truth about the game anyway.

I think once the Citadel came out a lot if players just stuck with one troop type due to the cost of the others stages. Checkout this link https://vikings.help/en/resources/troop_upgrades/ A quick look at stage 1 for level 7 troops and the cost is already very high. 

Why some players have done stage 6 I don't know.