I think the answer is that you should not worry about XP so much :-)
Regarding your analysis, I think the fallacy is that it is hard to talk about average nodes. Some posts tutorial in character could get recognized as valuable again and again; some throwaway comments that were cute at the time disappear into the murk.
I don't know any monk that says: OK, I have reached some number of XP; time to rest on my laurels. For most, the fun is in the process of puzzling out questions, helping others and showing off cool techniques. That fun doesn't stop at some predestined XP level.