Yep.
Training + paying engineers and technicians to physically repair RROD = LOL.
Microsoft (nor Sony) would never do such a crazy thing. Much faster + lower re-failure rate (due to unseen damages that might take time or be effectively impossible to test for in a time-efficient fashion) + cheaper due to no sophisticated labor process = they just replace the damn thing and call it a day.
I used to work at dell and they had probs with bad caps on their mb's as is well known in the industry. Replacement motherboards were not new - they were refurbs. Do you know how easy it is for a technician to replace some bad caps on a board? I've done it many times and I can assure you it's far cheaper than using a new board. The xbox would need a lot more work of course.
By the way, the re-failure rate with the dell MB's was high. Dell didn't care. It is well known that the re-fail rate for exchanged xboxes is much higher than for new xboxes.
I'm still doubtful that microsoft used new MB's when repairing xboxes. When I had to send in my zephyr xbox to be repaired due to rrod, the cross shipped xbox I got was also a zephyr and it was definitely a used unit on the outside. Also, the zephyr xbox was produced in 2007 and I didn't send mine in until 2011. I doubt that the parts were in production any more and the silicon chips were DEFINITELY not in production any more since they had moved two feature size nodes in that time. Since the xbox had so many returns (100% of the pre slim units are defective and will eventually die), there is no way Microsoft could have provisioned enough spares parts to repair then all without fixing old parts.
Also, keep in mind that there are companies in China who specialize in repairing bad electronics. MS doesn't have to train anybody to repair electronics just like they didn't have to train anybody to build an xbox in the first place because they outsourced all that work. Don't think that shipping costs are prohibitive either. It's cheaper to ship things in bulk to china than it is to ship things in bulk from one side of the country to the other.