I have 8 x XTR-RB's and use all my GTE, GTE B, XTEA 6000, 8000 and 10000 spools on them.
Mechanically they are the same as the old GTE, Shimano just changes some small componentry but otherwise 90% the same. Very happy with mine and don't see them disappearing of the market soon.
XTRA as said here is dicontinued but you'll have product support for another 5-8 years and a lot of the XTR-RB parts work on the RA as well.
When, in 5-10 years my XTR-RB's are retired I'll look at the newest range available then. Till then I'm happy and have a total of 32 spools all with different thickness lines and braids etc, etc on them.