Our RNP arrivals and departures work great when ATC lets us do them without "controlling". In Southeast Alaska or the Arctic it is common to be cleared for the approach while in cruise. That is when this technology shines, assuming that you gave the box some descent winds, ISA dev, and Alt.
I think it is a long long ways away from working in SEA. ATC just isn't capable of doing it in a high traffic area.
As to our pilots not playing along in VNAV. That won't be a problem. It is sad to say, but I have not flown with an FO hired after about 2007 that could plan his/her own descent. They do not know how to fly the jet in anything but VNAV. They have never been taught how to do a 3 to 1. Every descent is done in VNAV and boards out, or VNAV and power up.