"Where does collecting of cartridges stop?" It starts and stops with the individual collector. Each one collects what he wants. He acquires cartridges that meet his criteria for his collection; someone else's criteria is basically irrelevant. Hobbies are started for the enjoyment of the individual and there are many motives for it - investment (likely the poorest reason), the opportunity to meet new friends of similar interests (certainly one of the best reasons), increasing one's knowledge within a subject that has attracted his interest, joy in accumulating items in which you have an interest (pride of ownership, I guess, would cover that), etc. They are ALL valid reasons for collecting, although most serious collectors would agree that some are better reasons than others, although they would likely disagree on which is which.
People leave a hobby for many reasons as well, sometimes nothing more than that they have tired of a subject. In that case, it is not unusual for someone to start collecting something else. Sometimes it is for financial reasons. Many of us have had to part with a collection that we would have liked to have kept, simply because of the need to recoup whatever funds we could for something more important than a hobby.
Just my take on things.