The thing is you need to keep it in mint or very very good condition a used reg in so so condition isn't going to be worth much unless someone significant owned it or is super rare. Which brings back the question, of why pay more for something in the hope that it's actually going to be worth something later and you need to baby it...?
Because you are a very rare person, a collector of regs.
I used to have the collector mentality with a lot of things, but I have really moved away from it.
I am also very into motorcycles and I have a couple of vintage Hondas that are quite rare and desirable, one of which is a "milestone" in motorcycling circles and is one of the few old bikes actually appreciating in value with each passing year.
And I ride the snot out of each one. Soichiro Honda intended the machines to be ridden and used. Not stared at. Ones I couldn't ride in good conscience, I sold.
My brother bought a 1974 Honda that had 28 original miles on it. He was just tickled. Then he realized the entire value of the machine was in the fact that it was literally a time-capsule museum-piece and he could never use it without fear of destroying the value. So he sold it to a collector. He effectively owned a 1:1 scale model of a Honda motorcycle, and that wasn't his gig. Nor mine.
I am also into Porsches. Every year, the Porsche Club of America, (PCA), holds the "Porsche Parade", which is a nationwide gathering of Porsche fans. One year, in the late 1990s, near the end of his life, Ferry Porsche was attending a Parade and he was shown a entire field of Porsches lined up for the "Concours de Elegance", which is a serious car show where cars are graded on appearance, originality, and they must be maintained to absolute perfection. Low-miles and pampering is the key in the Concours. These guys park their cars on top of mirrors so they can see how clean the underside is without having to get on the ground. Seriously.
Anyhow, Ferry Porsche was shown a field of these immaculate machines and was asked what he thought about it. Wasn't he proud to see his creations so lovingly cared for by their owners?
His reply? That it made him quite sad. "Our cars are meant to be driven," he said, "not polished."
I agree with that. I drive the piss outta mine.
Regarding scuba gear, I have some "limited edition" or "rare" pieces of gear, and I dive the hell out of them. "Rarity", "scarcity", "collectability" or "value down the road" is a non-issue to me. If I break or bugger it up, I'll fix it, and drive on. If it's not fixable, I'll get something else.