For as long as they are needed, and spare parts are available - sometimes by cannibalising other members of the class. Rather than buy new locos, I think the operators are increasingly more likely to use "train" locos (e.g. Class 66) to do most of their own shunting where possible.
On the last point, we do have a modern contender in the form of the Clayton / Beacon Rail Class 18 - a Hybrid loco predominantly running off battery power but fitted with a small JCB engine. I think though, that it is a testament to their design that the 08s & 09s have lasted this long. Simple, almost bombproof locos really - if a little, at times, temperamental. But then again I've not worked with any small shunting loco that isn't!
I don't believe there is much, if any, interchange between rival quarries. However, what the shunters DO have to do, is move fully loaded trains (up to 2400 tonnes) out of the loading bays into the holding sidings ready for dispatch. This would be beyond the capacity of an 08.
