Age of masters

Seeing new releases using re-masters which to me means they use the original 40+ year old master and twiddle some knobs.

Do 40+ year old masters still have viable iron oxide?  Would high frequencies suffer? Or do I misunderstand re-mastering?
That's very true. But in some cases, they dare not run a decades old master tape with missing or peeling oxide, or edge damage on an ATR.

If the mastertape is damaged, lost or burned in fire all we need is original record (not a reissue from digital copy)
I have about 100 production master tapes in my collection - these would have been copies taken straight off the original stereo mix down by the record company and sent across the world to record pressing plants. Local record pressing plants would often cut their own lacquers, hence the variation in quality between different pressings.
If a record company decides to remaster an album, they will use the best available tape - multitrack session tape, original stereo mix down, production master - depending on the availability and quality of the different "masters". If they don’t have tape available, they will use the digital archived copy.
My tapes are all 30-50 years old and sound magnificent. But just like the record companies, I archive my production masters onto new 15 IPS 1/4 inch tape as well as a 192/24 digital archive copy. I play my dupes and store the production masters safely.
@topoxforddoc , Hi! How did you manage to come up with all those production masters? Must have been a great gig! Truly jealous:-) 
There were hundreds of thousands of tapes across the world, most of which ended up in a skip back in the late 80s/early 90s, as record companies moved to CD. There are a number of places in Europe and the USA, where people saved these from the rubbish tip. If you search hard enough, you can find reliable sources of tapes. There are also a few collectors, who share tapes.