Exactly, and Sasuke's.... in his teens? I wouldn't think Kakashi'd be going downhill just yet, but I wouldn't think his capacity would be growing substantially neither.
During one of the first episodes of Naruto, Kakashi explained to them about using techniques for best effect or some shit because people have limited chakra.
Keeping a large repertoire of techniques is important so you can use the most suitable one when the time arises. As for the point of learning "more powerful jutsus if he wasn't able to use them", I don't believe we've encountered a technique yet that someone has been unable to use because it uses too much chakra, only unfavourable because they'd get wasted.
In fact, wasn't that one of the reasons for developing the Forbidden Jutsu catalogue? Shadow Clones were banned from general use because it split the user's chakra in half or more, and misuse could easily get them killed.
Other ones we've encountered were sealed because they were "unethical", but there's no reason why other jutsus weren't sealed for the same reason as the Shadow Clone Replication jutsu.
If this last episode depicted things accurately, depleting one's chakra is fatal.
If it's not Isuzu-chan Mii~
Look, feel free to think what you will but i for one find it hard to believe that a kage level ninja went 3 years of training without improving himself in the aspect he was lacking the most