What happens to the GOP if Hillary wins 2016 and 2020?

[Asian Nazi]

They will change nothing because most people will still buy into the traditional narrative that "voter fatigue" is a thing that exists and that their victory in 2024 is inevitable.

Also while it is possible for libertarian Republicans to get elected (usually by downplaying their libertarian views), running on libertarianism is not a viable electoral strategy at the national level and will likely continue to be so unless America becomes a nation of 18 year old white boys.

[Asian Nazi]

Once all the Silent Generation and Baby Boomers die off in 20-40 years, the GOP will have to find a way to appeal with Gen X and Millennials. That's why I think a radical change in party platform is needed.

The GOP's problem is less a generational one and more a racial one.  The generation gap is only as wide as it is because Millennials are so much more diverse than the Silent Generation.  The gap between white Silent Generation and white Millennial isn't that big and is liable to close a bit, but the GOP has shown no signs of being able to win minority voters of any generation.