Right, but it's not like there was some policy handed down from on high that said that RPUs couldn't be extended, ever. I mean, if you think about why they would allow you to extend GPUs but not RPUs, there are a few obvious reasons:

1 - there were a lot more RPUs in circulation
2 - GPUs were perceived to be much more valuable than RPUs
3 - GPUs (used long-haul) had a fare class restriction, so they knew if they extended them, people would likely use them on W fares

But, when they moved to PlusPoints, none of these reasons really hold true, and I can't think of a good reason to allow somebody to carry over 80 PlusPoints -- which could be used to upgrade one long-haul G fare or four CPU-eligible G fares -- but not 20.