list thoughts on "unsupported" hardware?

I realize this isn't arguing about Windows patch mechanisms, but recently
realized I've never answered this issue to my own satisfaction... How long
do we keep upgrading and using network hardware once it's fallen off the
support lists? The Cisco 7500 finally went off back in Feb of this year,
as I recall. 3rd party upgrades, and used parts, are still readily available.

(Actually, does anyone have suggestions on vendors for said upgrades and
parts? I've noticed a lot more discounting than in the past, but usually
from vendors I have no experience with).

A client I've recently taken on happens to be relying on a 7500 for their
border. In reality, their current use could fit on a 2621/2650, though they
have been much larger in the past (there's a small pile of DS3 cards sitting
on the shelf). They're still relying on a single provider for connectivity,

So, does anyone have any thoughts on how long we should be letting our
poorer customers/employers live with products that are officially off the
support lists? Clearly there will be (i.e. IOS) image support for quite some
time. Is keeping (tested) spares around sufficient to justify actually
spending some money to fit the newer/larger images? Newer/still current
hardware seems much more a no-brainer, but advocating spending a thousand
bucks to avoid spending 5x that on a more current fire-sale item is a little
less clear, to me.