Can you practice a craft for years and years and still feel like there is more to learn?

I'd turn the question around completely.

Is there ANY subject known to (or invented by) man in which 100% of all there is to know, is indeed known?

Probably not, IMHO

There are simply too many angles on any given craft, whether it's working with wood, metal or playing an instrument like guitar to even approach mastering it all. 

Andres Segovia for example was said by many to be a master of the guitar in his style...and there is no doubt that he was one of the  best. However, if you were to have asked him, he would probably have said that he was still just a student of the guitar..... and that was only one style that he was even close to being a master of. 

mike_mccue

the foundational concepts of "craftsmanship" is that you constantly have to evaluate and then re-evaluate

IMO  This is the only way to be in everything one does and this rule applies to anything and everything.
 
Process improvement is a wonderful thing 

I read somewhere that Neil Peart started taking drum lessons again.  This would have been sometime in the last 10 years.  You know, after Neil had already enjoyed a good 25-year run in Rush, and being largely regarded as one of the best drummers ever.

I took a lot of music lessons as a teenager (on alto sax), and I became decent.  Did well in some jazz competitions, won some local awards, nothing big.  But the driving force behind my efforts was "there's always someone better".  If you think you've reached the top, and you stop trying to improve, then game over.  You stagnate.

This would apply to any craft, wouldn't it?

In everything I do--music or life in general--I have found that the more I learn, the more I realize how much more there is to learn.  For me, the process goes in incremental leaps.  I'll learn something or figure something out and then build on that until it becomes second-nature.  Then it will be a short while until the next big revelation occurs. 
 
 
RabbitSeason

I read somewhere that Neil Peart started taking drum lessons again.  This would have been sometime in the last 10 years.  You know, after Neil had already enjoyed a good 25-year run in Rush, and being largely regarded as one of the best drummers ever.
 
 
A lot of the jazz guys who teach at the college where I work continue to take lessons.  It's the only way to guarantee that you will practice daily.  You have to have a drill sargeant, or at least someone to answer to, in order to force yourself to do it with that kind of dedication.   Deadlines are great motivators.

In my experience anyone can learn; from a complete beginner sometimes, even if that's only how to become a better teacher.

I read 2 distinct subjects in this question..., and then the undertone. The craft itself and what it has to offer (theory) and the mastery of the theory through practice (skills).

At a fixed point in time, a craft may seem to have a finite amount of theory to offer and some may feel that their is no more to learn, but can you say the same about the skills? And as your skills grow along with your understanding of the theory, wouldn't it be normal to touch/transform the theory with your own discoveries through practice, hence growing the theory further, where it may actually become a subset of the original craft demanding a new set of skills to master? Isn't this part of human nature, hard coded in our genes? I believe that if theirs no more room for learning/growth/renewal/reinvention or desire for such..., I might as well be dead.

On a more non-esoteric note, I practice my instruments on a daily basis (just about) and it's not easy. When I was much younger, it's something that I had to do, being completely helpless in the matter (discipline was not an issue). Now it's a different story. And it's not because I don't love doing the music, but I have to admit that the flame, even though still burning, is not burning as hot as it did many years ago. I cannot play the music if I don't feel it, and practice helps me in this regard. That's the motivation I use these days to discipline myself. I still feel that there's also lot's to learn, but in my case not enough time in this lifetime to even make a noticeable dent in the mastery of this craft. There's always going to be more to learn and through practice..., master. And besides, I have no doubts whatsoever, in my case at least, that the journey is the real challenge and reward, all at the same time.

On the undertone...

I do believe that whenever someone feels like theirs no more to learn (regardless of the craft), that it is simply due to the fact that the interest in that particular craft has dissipated/died, which is the case for many (and that's a sad state of affairs especially when so many years of effort have been invested in learning the skills). But that's a reality that some of us face, sometimes more than once in a lifetime. It's not a terminal condition. It's part of the renewal process which some of us have the chance to experience more often than others.

I know what I need to work on.

Arranging!!! AAARRRAAAAGGH!!!!