You have asked one question, but the board population is answering 2 separate questions interchangeably. I will answer both, separately.
In a reg rental fleet: you want newish gear, you want to inspire confidence and comfort in your customers, newish gear does that. Most people who rent gear don't understand it well enough to know about parts being available from the OEM or if that is a negative or not. Most people who rent gear want to rent something that looks good/new and feel good about it. In a court case having newish gear will help you because judges and juries are like customers in their thought processes regarding the suitability of rental gear. Lawyers only care about what they think a judge/jury will think.
For diver's personal use: Who cares how old it is? Who cares if a shop can get new rebuild kits for it? If it was a safe reg when it was supported by the OEM then it is still a safe reg for you to use provided that whomever services it knows what they are doing. Most special replacement parts for regulators are available from Trident or someplace similar (Vintageregulator.com or northeast scuba...etc etc) the remaining parts are O-rings and they aren't rubber, you(or a competent service tech) can easily get VITON or EPDM replacements for the regs just like the OEMs do and use them without worry, they have a shelf life of 20 years but I doubt you would need it since they get replaced so often.
I own and dive older regs, some that are not supported by the OEM any longer, no issues. If I owned/operated a rental fleet of regs I would have newish ones in that fleet. I would also have recorded service intervals according to the OEM published recommendations, no exceptions. Personal and public usage are completely different.
On a personal note: Kudos to you for using your real name(s) integrity seems to be a diminishing attribute these days. I don't see a problem with it.
~Chris