SuperCell should introduce 6th builder to encourage players to play more for achievements otherwise players with master league or above loose interest about geming and cannot utilize their gems properly as they already have 5 builders and think boosting resources are not worth. Even if the builder cost is 3000-4000 gems , it will be able to change their mindset.
It's not worth it, TH10s can't even get all their builders into work, why would they even buy a 6th builder? I get your point since upgrading is fairly cheap on lower TH levels but considering this overall, including high TH9s and mid-TH10s. The standard TH10 player can't catch up resources to farm and at most, 2/5 builders aren't working.
4000 gems for a builder that you will rarely use is a waste, although this won't apply to lower TH levels as it's still pretty easy to farm. What I'm saying is that it's more beneficial to use those gems to speed up barracks and collectors as they increase resource income.