What trouble will I have when running a SAN past its support lifetime?

This is a exercise in planning and setting expectations (to your users, the business, etc.).

I'll use servers as an example. When I sell/buy a system, I plan for it to have a primary service life of 3-5 years. That's a pretty good metric for modern equipment since there's usually a substantial jump in technology in the interim and good justification to upgrade after that period. That's also when the failure modes of the systems present themselves.

Systems beyond that age are still usable, but the lack of parts and support relegate older servers to non-critical functions or use in clusters that can tolerate failure.

Storage has also evolved since the time your EMC was in wide use. I'd say that SAN storage has become more commoditized, with more intelligent caching and performance features. You're probably leaving a lot of performance on the table...

As far as keeping the old unit in use, you can, but why not rely on the new equipment you have? What do you expect to gain by keeping the older gear in place?

Things you will have problems with on 'old' kit:

• Code updates: vendors will rarely commit to releasing updates on older kit.
• Replacement parts: Spares will become increasingly hard to come by - sometimes you can use newer parts, but not always, because the speed/communication mode/protocol etc. gets updated. Newer SFPs stop supporting lower transmission rates, that kind of thing.
• Failure rate of moving parts: spinning disks wear out, so you will start to see the rate of disk failures increasing.
• Infrastructure compatibility: vendors like to change protocols as time passes. Windows domain controllers, for example, deprecate legacy encryption protocols.

You are also paying the opportunity cost of not upgrading:

• New toys are typically Bigger, Better, Faster. Storage doesn't quite keep up with processors, but there's some very nice capabilities leveraging flash drives, bigger memory caches, etc.
• hiring people with experience will become difficult.
• steadily increasing migration overhead when you do finally make the switch, because the migration paths to 'leapfrog' a tech generation are less well trodden.
• Some vendors offer trade-in deals, for much the same reason car retailers do.

I won't say it's a bad idea, but you need to consider why you bought a storage array in the first place. They're usually quite expensive ways of buying capacity - what you're doing is making use of performance over subscription - to get a better 'burst' with the same 'average'. Both at disk layer, and cache layers.

They're also more expensive because of improved reliability - 'enterprise' components with better MTBF.

Both of these things diminish over time. The former because the goalposts move, the latter because of wear and tear and availability.

So it's really more a matter of acceptable risk as anything else. For my production kit, the data on it is FAR more important to my organisation than the cost of replacement and vendor support contracts.

For my test/dev kit, I don't care nearly as much.

I would suggest you therefore present this a bit like a thin provision. It's not saving money it's deferred expenditure. You are still going to need to replace it. You are going to incur additional overheads as it ages. You are going to make your replacement and migration harder. You are also taking a commercial risk of hitting an unfixable fault. That would need vendor support, who will either point and laugh, or hit you with a ridiculous bill. Or maybe both.

But you might find the money you save in the meantime, offsets the cost, and that by delaying your purchase you get bigger and faster for the same money.

The bathtub curve is applicable here: http://en.wikipedia.org/wiki/Bathtub_curve

It very much applies to storage arrays. You can probably compare with a car quite nicely - as a car gets older, the cost of keeping it on the road steadily increases, as do the odds of it breaking down, and the trade in value decreases. If breaking down every few months and needing to repair it right then is acceptable, then you might well run an older car. But you wouldn't do this with an ambulance, because whilst the odds are the same, the consequences of failure and downtime are higher as well.