It'll be around a while yet!!! Particularly so in the UK GA environment.
How else would you fly an IAP to 03 at Biggin, 06 at Southend, 04 at Cranfield, 05 at Cambridge, Oxford, Gloucester etc....etc....? (Or any other aerodrome when the ILS is u/s)
I'm a firm believer that GPS is the way forward, even to the point that a Pilot III velcro'd to the coaming and a Maglite is probably a safer 'approach aid' than an ADF at night near a CB!!! The fact remains, that NDB will continue to have it's uses probably indefinitely and while it does, it'll still stay in the IR syllabus and on the 'mandatory' equipment list.