Jeeze there are 'assumptions' being made about F/O's in this thread. Whatever happened to NOTECH skills ;-)
For those who don't know or haven't had their career run this particular path, believe it or not there are FO's in airlines who, not only have significant command experience, but may have training skills too.
For instance I had over 8 years regional jet command experience, before finally biting the bullet and moving back to the right seat as, at the time, it was the only way to progress my career. (DEC options are limited for those without Airbus or Boeing time). Today I still have more command time than FO time and more command hours than some of the Captains I now fly with (and no I'm not a training FO nor do I wish to be one).
To state blankly that an FO cannot be a TRE and "nurture new captains" is just ludicrous. The right person for the right job based on skills and experience.
Having said that I can understand why some ego's might not be able to cope with that, it's a pity that we've not yet moved away from that particular problem in total ;-)