Have a 5 year old HT II XT Crankset and after a winter of sandy/gritty rides the shaft has worn excessively at the bearing interfaces.

Is this to be expected for something that age?

Assume I should bin it and get a replacement.

sounds like the bearings are ‘catching’ – the axle shoudl be an interference fit on the plastic bearing covers and take them roudn with it.

how much wear is there ‘feel it with a finger nail’ or see it with your naked eye?

either way might be worth getting bearing sorted to stop the problem reoccurring.

Both – it is very obvious visually but when you run your finger over it there is a pronounced ridge.

Very pitted and worn.

I can only assume that grit has obviously been getting in between the nylon and the metal shaft.

Most probably a right off, unless it is possible to get a replacement shaft.

are you running replacement bearings without the plastic spacers?

the plastic should be sacrificial in case of bearing failiure, ive got some deus cranks that the previous owner ran with replacement bearings and the axle has a slight groove where the bearing sat.

That sounds like your bearings have seized, or partially seized, and your axle was spinning inside the plastic sleeve. The axle cannot be replaced (it’s part of the cranks). I find it is worth keeping an eye on the bearings, and every few months pop them out, clean them and put fresh grease in; it takes only 15min or so to do.