This is a really interesting question actually, and it’s something that I have considered too. For me the biggest issue with using it long term is that it’s not sold in Canada and even when I cross the border it’s not easy to find in every Walmart and I don’t really want to pay $40 for it at the other parts stores.
But another reason is that it’s a bit more than M1 ESP, which has a lot better approvals on it and even if your car doesn’t need any of those it’s still a good indication of the quality of ESP.
Also, I know a lot of people here like to use a 0w30 or 0w40 and that’s something Valvoline doesn’t offer in R&P.
I think this is the main reason that I wouldn't use R&P as a "forever" oil. While R&P may be a very good oil for cleaning, there are other oils out there that have went to great effort to be formulated for, and obtain manufacturer approvals. And it most instances, these approved oils are no more expensive, often less expensive, than R&P. For me at least, a motor oil that has been formulated to meet or exceed manufacturer specifications, particularly one that is licensed/approved, will almost always be my first choice.