I think I may know the answer to this question but out of interest:

How many would take the chance on reconditioned turbos from one of the many online/ebay vendors?

I have replaced the turbos on two 3.6 tdv8 l322 previously and although not a pleasant job there isn’t the factor of labour cost. Most garages won’t install them due to potential warranty issues.

If looked after regular oil changes, mechanical sympathy and no boost leaks will the lifespan really be that bad?

I have looked online but can’t find anyone asking I am assuming this is due to the job not being very DIY friendly.

The price being lower, reflects the high probability you will be fixing again soon, you of course know that.

Out of interest, how did you change them, ramp and drop subframe, or driveway?

I have access to a 2 post ramp however the process I used could be done the same way with jacks and axles stands.
Lift the Range Rover until the wheels are off the ground and the majority of access is through the wheel arch and underside.
Drivers side is pretty self explanatory as you can see it before anything is removed.

For the passenger side remove; wheel, wheel arch liner, shock absorber, hub and drive shaft. This gives access to the engine mount.

Trolley jack under engine/subframe. Fully remove engine mount.

You can then move the subframe up and down for better access.

Turbos can be replaced with this method in a weekend.

My ramp is in an old barn with a low roof so I can only lift the Range Rover 3-4ft max.

I have replaced a decent few turbo cores for the Volvo 2.3 engine without any problems later on. For my own car, with a pretty non stock amount of power i have used a replacement core and it is still going strong after about 100K km. As long it is balanced right and oil is up to spec i cannot find a reason not to use a replacemnet core.