My 24-70 is probably close to that age and still working great! I have no plans to update to the new model, until I wear out what I have now (well, maybe sooner... the new one appears to be considerably smaller and a little lighter, and I ain't getting any younger.... but I'll wait for the "street price" to drop a bit... and maybe a windfall like a lucky lottery ticket).
The 24-70 "Mark I" does seem to sometimes need a trip to Canon for calibration... Mine hasn't yet, but Lensrentals.com noted this with their many copies of the lens (that probably see pretty hard use and spend a lot of time bouncing around in the back of a UPS truck). If you run Micro Adjust on the lens and have trouble dialing it in at the two focal length extremes, it might be time to send it in to Canon.
One other thing to think about, eventually Canon will no longer have parts to repair any discontinued lens. It varies a lot, though. Might be ten years... might be only three or four (the 200/1.8L for example, has been hard to get the AF repaired, if needed, from only a short time after it was officially discontinued).
I really can't say what the price for a 24-70 should be... Shop around in the Classifieds here to see what people are asking for them. Check the used depts. at B&H, KEH and Adorama to see what they're asking. Do a search for ended auctions on eBay and see what people are paying for them.