He stays 3m fine but the Gold Cup through the years has a whole host of great chasers that stayed 3m fine but always found the extra 2F beyond them. He is in the race they know they can win as opposed to the race they hope they can win.
That being said, personally they should have given him the chance to prove definitively whether the 3m2f trip is too far or not, especially given it is a rather open looking renewal this year and the fact that at 9 he will be (statistically) too old next season.
These type of things are as a direct result of the Festival going to 4 days and them now providing pretty much every type of horse an opportunity to win a G1 at their specific distance (except for the G1 2m4f hurdlers……which if they go to 5 days, I fully expect that oversight to be rectified).
At the end of the day there is no point blaming Mullins as the owner always has the option of overuling what race the trainer thinks the horse should run in and JP in particular I very much doubt would be shy in making his opinion known……maybe he actually thinks that lightning will strike twice with Inothewayurthinkin. 