In my opinion this is true for most learning processes of "interesting" things. You can always learn something new.

It is important to realize that the amount of new things one learns about a topic is not constant in time. Typically a lot of new things are learned initially, while over time less and less new insight is acquired. To put it in mathematical terms: initially the learning curve is pretty steep, but after a while it gets more and more flat. Or to rephrase it in terms of Pareto's law: the first 80% are learned in 20% of the time, the remaining 20% take 80% of the time (except that it shouldn't add to 100% in this case ;-)

For me this implies that after a while I've to switch to something new to learn in order to keep learning sufficient amounts. Is it worth to invest a lot of effort for the remaining 10% or 5% or 1% one doesn't know yet? This depends on how proficient one needs to needs to be at that particular topic, but in general I find it's more worthwhile to know, say 75% of a lot of things rather than 95% of just a few. The good news is that the former can be achieve with about the same effort as the latter.

Just my 2 cents, -gjb-

PS: any numbers cited are purely for illustration, obviously ;-)