I think it's difficult to stick to any rw guidelines while operating in the sim regime. So while rw airlines may upgrade their planes to some FMC for obvious reasons (they don't fly planes for the fun of it), the sim folks don't have to do this and can pick whatever they want to. As long as it works out.

You can run close to the rw, but you don't have to. That's the beauty of the sim. First of all, it's about the fun of it, so you can fly like in the times the 727 was new or you can try to get into modern things and work around some instrument quirks for example.

I doubt that many rw airlines operate a B377, a 707 (few are left) or Concorde these days, but I'm pretty happy to be able to driving those things in the sim, with some not too fancy equipment.

So while I would like to see a well done 727 retrofit with a sort of FMC, I may remind the folks about the loss of enjoyable workload that comes with it. As it shows, that sim fanciness often comes with another loss, the one on the basic 'how does this thing actually work' aspect.
That's a thing happening because the sim allows you to learn backwards, so quite some of us may have flown fancy equipped birds first and then came back to the steamliners, asking how they actually operated.

Each to his own of course, so maybe somebody adds some FMC-like stuff to the sim 727. This would give you VOR / ADF, cheated FSX GPS, the INS and then some fourth option. Freedom of choice.  :)