Also, in my original comment, I didn’t point out the fact that once they are fully upgraded, you might want to switch to a different class “just because”. Maybe for GW, or because of the other troops you’re using, or a different weapon or whatever. But, nope, can’t have that. Not for free or even remotely cheap.
I mean, come on, we have plenty of new Gem sinks to suck us dry. Do we really need an old one too?