You just did: you said that “inactivity” in your proposal is what the game currently uses. And that makes your suggestion problematic for the reasons I described. That “inactivity” detection is beaten simply by jiggling the mouse every couple of seconds. Right now, none of us needs to use a script that would automatically do that for us, but that script would be easy to write and distribute, and that would happen as soon as your suggestion made it desirable to idle on a server overnight.
An upkeep system is a system that requires you to do something in order to maintain your presence in the virtual world. For example, having to build a “tax collector’s booth” on your property, where you have to pay taxes to keep your buildings from going poof. (That’s not what I want, that’s just an example.)
An upkeep system, by its existence, incentivizes certain activities in the game. For example, if you had that tax collector’s booth I mentioned above and the taxes were paid in gold bars, it would incentivize collecting gold.
Your proposal incentivizes idling on the server. It’s a bad incentive.
Already did, a long time ago. That’s just my own idea for it, but it will show you what I mean in general. I wouldn’t oppose a different upkeep system, as long as its incentives weren’t badly aligned.