In a sense, we are already buying ebooks on a "donation" basis.
I've used before the public radio analogy. Anyone can listen for free, so why does anyone donate? Because a certain percentage of people recognize that without support, the programs they like go away.
My book is DRM-free. Theoretically, a single copy sold can become a million or a hundred million ebooks overnight given away free to family and friends in a never-ending spiral (until everyone has a copy).
Yet, my sales continue to go up every month. Why? Because word of mouth is good and there are many, many people out there who do "donate" to my efforts.
I don't currently make a living from writing, but I do supplement my day-job income with it and plan to supplement my retirement income with writing, as long as I can make
something reasonable from it. Otherwise, I'll take these fun ideas I have and just sit on the front porch and think about them and entertain myself alone.
But I believe that if you have good ideas and can write decently, enough people are out there to support you. You just have to find them.