Lifespan of old (Canon) lenses

With Canon full frame cameras (and Nikon, I believe too), the WACP-C and WACP-1 from Nauticam require old lenses that have been discontinued. For example the EF 28-80 f/3.5-5.6 V USM was released in 1999. The EF 28-70 f/3.5-4.5 II is even older. Any thoughts on the expected lifespan of these lens if they are purchased now? Put another way, for the people that have purchased these lens over the last three years to use with these water contact optics, have you experienced a failure in the lens that required purchasing another lens so that you can continue to use your expensive piece of Nauticam equipment?   

I believe that if you find a good copy, with "normal" use, they should last for quite a while.  Perhaps decades.  In my experience, lenses become worn out from extensive use,  (thousands and thousands of shots) or suffer from extended periods of non-use, and of course, can suffer from mistreatment or damage.  As I type this I am looking at the 55 micro on my Nikon F2 , that I purchased either in the UK or Germany in 1985 or 1986.  Works like new.  I have some others around as well and have not had any failures.   I suspect zooms might be a bit more prone to problems.  Another threat is fungus, if you live in a humid place. Lenses should be stored where temperatures and humidity are not extreme, generally.

Used in dive housings, I would think the lifespan would increase.  A relatively dirt and dust-free environment without direct knocks and bumps and such.

It might be prudent to purchase an extra copy or two, just in case.  Not sure where you are but at least in the U.S., MPB, KEH and Roberts Camera are pretty good sources of used stuff, with decent return policies and warranties.

I should look; I might have an old canon zoom around somewhere that I should sell.

I've owned SLR's since .., forever (1980 at least) and DSLR since 2005 or so.   Lots and lots of (Nikon) lenses.

I've had exactly one lens just fail on me - an 18-70 kit lens with a D70s that failed autofocusing at 1 year.

I also damaged an 18-200vr by dropping it.   Sent it to Nikon for repair, and ... they lost it!  (Ended up sending me a new one for free).

And I had the start of the zoom grind issue on a very well-used 24-70g that I carried around the world for 5 years.

Lenses don't fail much, but when they do it's usually something to do with the electronic bits, mostly autofocus.   I really would not worry about it.  If one of those lenses fails, you'll find another used one at even less cost, probably.

In my collection, I have lenses dating back to the 1950's that still work.   Of course there isn't much to fail in older manual-focus lenses.

16 hours ago, Draq said:

Another threat is fungus, if you live in a humid place

A good point by Draq. It's worth checking if you can that helens hasn't got "misty" spots - fungus. After  year running a dive resort on Lembeh I had to sacrifice two lenses through fungus: a 60mm and an 80-400. Expensive and annoying. Humidity does not help lenses!

Those older lenses are mostly reasonably well built, the solution to failure is to source another copy, there are reported to be a great many out there.  The Canon lenses do have the advantage that they all use an electronic diaphragm and built in focus motor, so will continue to work on an adapter for RF mount.

Thanks everyone for the response! Good to know that there is optimism about their longevity.