Re: It depends on the app
For complex things I agree, it needs tuning to the device. For simple things it just needs to fill the entire screen on a phone and run in a small window in the corner on a desktop. I'm thinking of the sort of utility which people used to code as 'gadgets' (before MS effectively killed them).
Then there is the (admitedly niche at the moment) case of devices like the Surface Pro. These provide a case for more complex programs which can adapt themselves. It'd be nice to not have to have two versions of the same program and instead have one adapt itself depending on whether it was in tablet mode with just the touchscreen or pllugged into multiple external monitors with a mouse connected.