Suppose everything you chose to do related to your language comprehension was similar to that lecture, and once even that had become easy to understand, your daily activities would then suddenly switch to something else that kept you at the 50-60% comprehensiuon level?
Repeat, repeat, repeat until you are a grey-haired old man nearing the end of your japanese speaking life and still enjoying the same rough success rate as you did when you spoke your first word. An ever scaling world means that no one gets to really become an expert at anything because the goalposts will just move to compensate for any gains in knowledge.
Umm, no. In an ever scaling world, you are constantly facing new challenges to overcome that are just about out of your reach. That's how you succeed in anything.
If you get to the point where all actions within a given activity are (pretty much) guaranteed of success, you stop improving. And, most people get bored and move onto something else.
It's only through constant challenges that anyone improves. You can play scales until your fingers bleed, but, until you actually start learning new songs, you won't improve your piano play.
See, the problem is, you're trying to say that all challenges are the same. That just because my success rate never changes, that must mean that all challenges are identical. But, that's just not true. I master a particular set of skills, then move on to the next task of attempting to master a more difficult set of skills.
Or, I stop and stop improving.
It's no different with any skill. You start with the basics and then, throughout your entire career you continuously attempt more and more difficult things. Depending on what you're doing, it is possible to master something (I can chew with my mouth closed most of the time after all.

) but, most complex activities have a never ending learning curve.
My daughter's fifty-five year old calligraphy teacher is still learning new techniques from her eighty-some year old teacher. And the older teacher says that she's still learning new stuff too.
Learning never, ever ends.