Something like that would be disastrous; some sort of inactive account usurpation system is something we brainstorm often. There are no simple solutions that are not easily gamable by squatters (a single scripted free vote every 6 months by squatted accounts defeats the system you just described) or unacceptable due to not sufficiently respecting property rights.
Thresholds regarding account balances and such would be essential to such a system. We are actively thinking about it. Such a thing will not be soon, in any case.
Glad you are thinking along those lines. A comprehensive program would be hugely difficult for the reasons you cited. However, I wonder if just targeting the low-hanging fruit: say accounts which are a year old and have never posted or commented? Could clearing those out be an easier task with less cascading effects? Just brainstorming ...as us amateurs do so well...ha! :)