Maybe there is another way: Diversify possible builds for each army. If, say, a pure Stonehorn force becomes viable, people will be interested to buy more of the existing models. When hordes became a thing, people had to buy more basic troops etc. That’s one reason why I think artillery is coming.
I‘d also be very interested in the number of sales for old and new armies. I can imagine that for someone new to the hobby it doesn’t matter, since everything is equally new to them. Many veterans may be more interested in adding to their existing armies. And those who come back after a pause may still like the old armies more. Thus, I wouldn’t be so sure that new armies necessarily sell better than new-made classics.