The Devs will not do that. It cots into their revenue because people will not be using as many Gems to open up bank slots. BTW, I do not buy blue medals on Fridays or any other day except at the Fair. With the rate of failure of the upgrades the cost of 10 Gems per medal is not cost effective vs the benefits that you get if you upgrade as far as I am concerned.
I agree benefits are not that great with blue stars. Once you go up to purple stars, the benefits are very big and noticeable. I would say, it is what makes or breaks the final fight. Who has the best stars and saints. But stars at lvl 13+ is going to give you significant dmg and defense boost.
Just getting to 9 blue stars is a challenge...