I guess that's why the original ramps board was so popular - you just plugged bits in when you wanted extra features.
Have to admit I really struggle to see why the duet boards are so expensive.

As far as the pi goes - if I hadn't found mini itx board with ram and cpu for less than a pi b costs, I'd most likely have thought about going that route for the laser cutters server setup. Though as main software is currently windows only - maybe not :-)
But given that I do 90% of my prints from sd cards.
Just don't see the need for a seperate server for the printers.

Guess it depends how many you have and how much use they all have.

All my printers are still all original electronics, I have no problems running them much faster than most people seem to do. At some point I might do something with the i3 as the firmware (marlin) is truly bloody awful and as it won't just flash, to change it would be a lot more hassle than dropping a new board in.
But other than that. Why change what works and does what you want it to ?