it promotes activity, yes
I think I used the wrong term, what I meant by gameplay is "variety"
for example in an MMORPG, I like playing long range, so I choose Mage or archer
but others like melee, so they choose swordie or brawler
in CoC's sense, its like (with trophies in mind):
"I'd like to prey on weaker THs and dead bases at lower leagues, I guess I'll drop to 1k trophies..."
or
"I'd like to push hard and have the highest cups in the world, I'll push to champions!"
making higher lvl TH be bound to higher leagues limits their options
btw, I didn't say your idea is bad
