You cited something you do, which is already a bad idea, and then said you'd like it better if the game forced you to do something else.
Is there an "in my opinion" in there somewhere, or are you speaking in absolutes? As in, "It's a Bad Idea. Period. End of story."
I don't see "How much would it screw up game balance if players were required to upgrade or disband units that were two eras behind the current era?" as my saying that this would absolutely be a Good Idea. Rather, I was thinking, this
might be a Good Idea, but I wanted to see what others might have to say on the subject.
As a
game, leaving the mechanism stand as it is literally doesn't change anything. It is as it was. It's no more unrealistic or unreasonable than saying bishops move diagonally in a game of Chess. But as a game that is at least somewhat
modeled on Historical Reality, the idea would move the mechanism a tad toward that Historical Reality. Would it unbalance the game? I don't think so as ALL of the players would be working under the restrictions of the altered mechanic. What it would do is that it would occasionally reduce the players' Military Power, either by requiring an expenditure of funds to do upgrades, or by being required to disband an obsolete unit. Understandably, players do NOT like to have their Military Power reduced. But then again, do they
like having to do a large number of other upgrades, just to stay competitive with the other players?