I haven't ever hit FPS death, when does that usually occur for you fellows? I'm currently at year 14 at my fortress and all is good, with 231 dwarves.
My idea of FPS death is pretty much below 10 FPS, which I think is pretty low by most standard. My normal fortress goes around 50 to 100 ( cap ) FPS, which seem to be low for most players I've seen too. I find that as long as dwarves gets to work at reasonable pace or even when I have something to read or look at, I don't mind low FPS too much since the game'll pause when something important and immediate happens. I've stopped bothering to enable FPS display when I started 34.11 and found I've been happier to enjoy my dwarves working without being menaced by the idea of slowness.
I probably would disable idle display, but I don't mind having 90% of my fortress lazing and chatting it up very much

Though I usually 'abandon' fortresses by just never coming back to them out of boredom or for new version long before FPS actually becomes intolerable for me. I does have a few files for older version with multiple worlds and fortresses on each of them, since I like to make only one fortress per world, unless I'm experimenting.
I think my current fortress' going on 25 years with 80 dwarves and more animals, haven't breached first cavern yet since I keep trying to move things around or decorating rooms. No goblins in this world, either, so I'm not worried.
I found it interesting to read (in this thread and others) that so many players wait a long time before breaching the caverns. I'm always at the magma sea by year 2. When I hit a cavern, I stubbornly construct stairs to the bottom so that my main staircase won't be crooked.
And yes, I don't quite grasp the caverns thing either. It astounds me when players have been playing multiple forts without having ever breached the first cavern.
I'll admit that it's mostly because jobs needing fuel isn't very important to me, so I don't bother to drill down very quickly. And what little I does need are pretty easy to fill out using charcoal or imported coals. I'm same in that I'm stubborn about building stair straight down, that does slows things down a bit

I'd probably drill down more of I bothered to embark in more dangerous areas or where there're no trees, I figure. I at least know I can hit magma sea within a year if I really need to, since I've done it a few times for practice. Aquifier and straight staircase can add up to a couple years for me, though.