I agree with the 20 year estimate. There is a lot more inertia in the system than most would believe. Avgas is all over the world, including the most backward bits of the 3rd world, used by countless thousands of little planes flying between farm strips.
It's also going to take 20 years for Thielert etc to come up with a proven reliable diesel engine, so that should work out just right.
The price will keep rising, but the cost of fuel is not an overwhelming proportion of the cost of flying.