I'm not sure why anyone thinks "retired" forts will be simulated at the Fortress Mode level of detail, including things like mechanisms and exact positioning of dwarves. Your computer is not powerful enough to run two fortress simulations at once.
This issue, I think, is not that the fort needs to be simulated while you're running another fort except in high level detail. The issue is that it needs to be simulated correctly if you visit as an Adventurer. Basic things like: What state for various levers has the drawbridge up or down so you can actually get in? If there is an airlock system, what levers have to be pulled to open the inner doors after travelers have entered through the first set of doors?
It's those more complicated structures that Toady is concerned about. Players do all sorts of interesting things in their fortresses, and sometimes there isn't a decent 'default' state to leave it in that'll keep the fortress working properly if it's loaded during visiting. We can debate about how much of an issue that is, but from what Toady has said he would like these issues to be handled properly.
This answers a nagging question of mine I wanted to bring up here, but didn't out of fear for what Footkerchief would do to me.
I had only thought of fortress mode, and figured "Who cares whether or not your magma trap or airlock works? You won't be going there, it just needs to simulate battles and trade, and the caravan arc will take care of the latter, world gen already does the former! Where's all this mechanism stuff come in?"
I never considered retiring a fortress and then going into adventure mode, mainly because I played adventure mode once and freaked out at the learning curve, which seemed even steeper and crazier than fortress mode (to me). Obviously, this creates a whole slew of problems that I'd never imagined due to my narrow vision on the game as a whole. Thanks for pointing it out.