I think there's a number of factors at play. Dieselgate, the cost of running 2 teams and only one can win, so one or the other is always losing. No prizes for second place etc.
Another angle is that Audi has been putting a lot of work into developing synthetic fuels over the last few years.
Maybe that is where they're headed next, as it has grown from an idea to a proven concept. Garage 55 entry perhaps, as in 2003 & 2004 with Team Nasamax running a pure bio-ethanol fuel, but this time with pure synthetic fuel?
MG Mark