I like this idea in general, I think that there should be some sort of mechanical recognition of longevity, whether it's a returned soul strand, or x quest-hours worth of xp, or whatever. I would caution that it should be structured in some way so that it doesn't encourage behavior like not logging on for a month in anticipation of getting that reward.
What is the basic unit of time measurement, and how many hours per unit is considered "active?" For example, if the basic unit is a month, how many hours per month in-game counts as an active player? Can it be broken down into hours like that? If it can be broken down into hours like that, then could the total hours per year be used instead? Could it then be a measure of a certain number of hours required rather than a certain number of months? If it can't be broken down into hours like that, then what objective measure would be used to determine activity?
The loss of a Soul Strand is by no means an OOC thing.