There are around 40 yard shunters (made up of sorting sidings and intermodal terminals) plus about 80 depot shunters (mainly TOC & Depot owned) still working.
There are a further number eg Foster Yeoman , working at rail-connected industries.
If you consider that the likes of Freightliner / EMT/ Alsthom / First Group / GBRF etc. most probably picked these machines up for peanuts, it's highly unlikely that they'll be looking at investing large sums of capital expenditure on a shunter replacement policy.
In the case of the TOCs who own these machines, their franchise terms could not possibly justify a 15/20 year leasing position.
As I think there are a fair number of 08s available from leasing companies (BARS,HNRC ?) at a fraction of new cost , the replacement policy will probably be one of Mend,Beg, steal,borrow and Main line including dare I say 60s !.