It's really not hard to clean up the edge on older pistons. This gets rid of the creep, which is why 90% of them get replaced. A trained "tech" reading the manual is going to read that the cure for IP creep in a MK5/10 with a new seat is "replace the piston", then read in the SP literature that the part is no longer available, then tell the customer that the reg is dead. A DIYer with an adventurous spirit and a bit too much time on his hands would lap sand the piston edge then clean up the edge with micromesh wrapped around a round chopstick. It works great; in fact, it even sort of works on the creep-tastic MK20 brass tipped piston.
There are some parts that SP no longer has. Try finding a diaphragm for the pilot or air 1, for example. But, you'd be surprised what you can keep going, especially with the right friends!
Of course, this is exactly how vintage divers have flourished into an active community of restoration, so posting this on the vintage forum is a little redundant.