That's sort of a non-question in my eyes.

I don't think people *only* improve when content requires it, they will just already be playing as well as they can, and if that's not good enough they will either just not do the content or wait/slog through multiple groups until they can be carried through it. Some folks have "improved" all that they can already and sometimes that just will still not be enough.

Sure, there's always going to be people who don't give as much of a shit and very well could "improve" but I think in the case of most people that have already been playing the same class(es) for a year or more, there isn't much room left for improvement in terms of what they are actually capable of (which is not to say that everyone is potentially capable of doing the hardest content, because sometimes they just aren't). The only way they will see "improvement" from then on is through the obtainment of higher level equipment.