Okay, I got it. That makes sense. I think there would be no shortage of optical problems if the camera had pixels the size of the longer end of the light they're collecting! And I don't even want to imagine the S/N ratio... I read about Lytro recently, it was fascinating! Maybe I'm being sentimental, but to me that would feel like "faking DOF"! The significance is huge, but it would feel so different if I had to use it in practice. Personally I prefer everything to happen optically that can happen optically!
You eventually reach the point of diminishing returns with sensor resolution if the lens is the limiting factor. Now, it doesn't matter how good the lens is...if you need to use f/8, you need to use f/8, and you'll never get more than 86lp/mm even with the best lens and the best sensor humanity is ever capable of producing. The only option at that point to achieve more resolution is to start taking more radical measures. Use f/4 and stack for focus. Maybe build a camera capable of always using a lens at it's fastest diffraction limited aperture, and use clever post-lens optics and software algorithms to produce whatever depth of field you need at the resolution of that maximum diffraction limited aperture. This is kind of where Lytro is pioneering something new. Their concept was consumerized, but it is possible they have the foundation of the future of ultra high resolution photography in their pockets (I don't know for sure, depends on exactly how their technology works and how applicable it is to different kinds of cameras.)
With Lytro it does happen optically. There is actually a special optical array in front of the sensor. They do longer exposures, and over the duration of the exposure time, they are actually gathering information in "three" dimensions. A lytro image is not just a bunch of pixels in two dimensions, it actually contains more information that allow their software to do it's thing. It isn't just software trickery, it is a combination of optical ingenuity and software algorithms that achieve the ability to change DOF in post.
Lytro is a limited application of the concept, though. If you play with some of their examples, you'll find that there are a number of discrete options for DOF, it isn't really a continuum. Improvements on the technology could make it more effective, bring in enough information that you could indeed have more of a continuous three dimensional field that you can tweak in post. The raw data file sizes would become considerably larger, however as time continues to trudge on, processing speed and storage capacity is improving considerably (i.e. CFast 2). I don't think that the Lytro concept would ever become a mainstream, frequently used thing...it would be one of those more niche options for people who really need it.
And there are actually already some options to solve some of these problems. Not quite the way an infinite field lytro-style device does, but tilt/shift lenses can be used to great effect to control your focus. You can either constrain DOF, or expand it such that you could photograph a landscape scene at f/4 or even f/2.8 and have the entire depth of field in focus and at high resolving power. Again, though, this is a purely optical solution, and as such, you tend to pay more for it, especially if you need the capability at multiple focal lengths...so a lytro-type solution could still offer something in a cheaper package.