That's entirely wrong.
I purchased my first Scubapro MarkV (that is a MK5 + 109) in 1975, and in the next 5 years I purchased other 3, so both I and my wife have two each.
I always serviced them personally (after attending the course at Scubapro Italy in 1977, when it was time of first servicing my first one), at intervals ranging between one to five years, depending on usage and conditions. We are still using them now!
And they are still performing better than most more recent regs, having applied all the updates that SP released for the following MK10 (concave seats),156 (fully balanced poppet), and G260-S600 (S-wing poppet).
This kind of regs (full metal body both at 1st and 2nd stage) were designed for lasting forever, of course servicing them properly, using the proper tools and skills, and replacing all the parts subjected to wear at the intervals prescribed.
Some more recent regs, with plastic body, eventually degrade so much that it is safer to replace them.
I am not the only one still using those classic regs of the seventies, all metal, and would never replace them with those newer plastic things...
Here on SB you will find a lot of people still using them!