So let me get this straight. Sauber prefered to have an old engine in 2017 because changing the layout would be too much cost and hassle for them. Even tho it was only an ugrade coming from the same factory they thought they would gain more in the chassis side. But because RB is such a massive corporation they would rather build a completely different rear in 3 months, adapt balance, space, cooling etc. They would also keep on paying Renault for this mega expensive engines whilst also paying Honda and I guess some compensation to Tag Hauer (I bet they won't be happy). And all of this to change for an engine that has ZERO proof to be better than the Renault and never was. Even if test bed numbers are good Honda has a track record of getting in trouble when hitting the track, so does Renault but at least they're already paid.
Or... Could they wait and compare engines between them and Toro Rosso, with all the data they gather it will probably will be very easy to compare strengths and weaknesses, see how both develop and how reliable they ar. Then in 2019 change to Honda using the information they alraedy have about cooling and integration from STR. Probably even move some engineers to the mother plant.
I actually would like the chaos of what you're suggesting a lot more but I just don't think it's very plausible