If they never use their top engine mode then the wouldn't be able to develop or improve it for the following seasons. I suspect Rosberg had everything turned up to 11 for the start of the Sochi race, the way he flew past hamilton despite having a poor start.
Mercedes were always targeting this regulation change for their attempt at the title. They really struggled with tyres the last couple of seasons, how much effort do you think they would put into ensuring they understand the issue and avoiding it from their shot at the title? It was clearly something pretty fundamental to their design concept which is why they couldn't fix it last season, but with fresh regulations...It really doesn't need a conspiracy theory.