I've not played much RM compared to my D&D days, but I remember learning the original RM2E just as the Standard system came out -- so I ended up getting the new books and was pretty happy with the changes. Then my teenage-years came and went, and I managed to get rid of those books:(.
Years later, when my interested was peaked, I looked and found RMFP and, even though it was supposedly just cosmetic or a bit of a trimming, I wasn't too happy. More years later, I started to look at RMC, but I seemed to drift back to the wanting my RMSS books back; so eventually, I was able to obtain them.
I don't know of any of the proposed changes for Unified, but I'm just wondering if there is enough "good" changes for most RMSS users to want to upgrade, especially when, for a time, just getting the original books was difficult. Probably whatever kept me to RMSS may keep me with it, though.