You're water isn't corrosive otherwise you would have already replaced components. I'd say the life expantancy is 10 to 40 years, what's yours
Yes, vendors still supply replacement parts for your existing system but what worked then isn't necessarily the best today. When you do need to replace components I suggest that you consider installing a Pside-kick http://www.cyclestopvalves.com/products.html. No need for a large tank and you'll have constant pressure while taking a shower. You're wife will love it!