The thing is that 80% of the usefulness of learning is that it's learning practice. This is why you sit for so much crap in K-12 -- you're learning how to learn. This rings even more true for techies who are always chasing a moving target (nonono, object-oriented CORBA RPC, nono, it's REST, it's Kernel SVM, it's Convolutional Networks....). The effort is (almost) the whole points.

If you don't keep your learning organs stimulated they will literally wither and shrink.

I agree with you but I don't think we should make things unnecessarily hard to understand if they aren't so. Do you? There will always be difficult things to learn out there but I don't think we shouldn't artificially make all things difficult if we don't have to.

   we should make things unnecessarily hard to understand if they aren't so

Who said otherwise ? I am just saying that if you as a teacher are concerned about making people/students more clever, you need to think both at vulgarization but also at the meta level where they need to actually make mental effort to incorporate ideas, maths, methodology, calculus, visualization capabilities and in general self-motivation

If you consider the brain like a muscle, then "leaving mental effort for elsewhere" doesn't really make sense. If anything, it's making it more resilient for future work.

It doesn't mean you stop learning with Markov Chains...it means you move on to the next more difficult topic.

Or you wait for the awesome teacher/youtuber to make the next topic easy to understand and grasp withing 20 minuts instead of going on on your own