Finally my point of view.
To buy a bigger sensor for legacy lenses is not really an argument here, since it's not really a technical reason, and is valid only for us, old photography lovers who already have a bunch of old lenses. Also we love FF, because we are used to it. It seems correct. It's an psycho-sociological effect called asymmetrical insight, wicth simply states: "Our way is better because we, a established "familiar" group(for us photographers using 35mm), do it this way. The others are just wrong"(and we have the audacity to point out problems for the other ones, feeling that we just know it all better, and that's why it's asymmetrical!). It's pointless to say that if fells right, or anything.
There's no correct sensor size, or field of view to lens. They are capable of cover a certain image circle, and that's it. Theoretically, any non fisheye lens can cover 180 degrees field of view with an enough big image circle(infinite, to be precise). Perspective does change, but correctable with special designs and post, to look the way we want. Also in the old days we did not complain that our MF lenses did not have the "correct" field of view on 35mm film. We did not use nikon lenses on canon cameras(usually), since it did not make sense.
APS-C has these basics pointed out limitations, but, plenty of good characteristics. Lens designs are easier for small image circles, even in faster designs, so a 23mm APS-C only, is very likely to cost less than an equivalent 35mm FF, in the same lunch date. They require less technique to be built, and controlled(it's harder to make big sensor with live view, for instance). Also, although nonexistent, one can builds(with less effort compared to 35mm format) multi-aspect ratio sensors, for squared, 645 or 16:9 formats.
Today we do have a limitation of lenses of aps-c, specially primes for dSLR's. But its a market problem. Many of us love to shoot with primes over zooms, but not younger first buyers of dslr's cameras, before talked into primes for they're "advantages"(asymmetrical insight again). So manufactures prefers make zoom designs, for small sensor. And they do primes for old users who get full frame(and sometimes charge a lot for them).
Finally, I don't think anyone can tell the difference of FF and APS-C. Not comparatively at least. I mean, using two cameras, one full frame and other APS-C, I take one photo with each, using different lenses, in different situations. If I show you these two different photos, can you tell witch camera I used in each occasion? I know I can tell film MF from film 35mm most of the time, since the quality is blowing away different. But it's so not the case in digital APS-C vs FF.
Camera manufactures move to full-frame for the simple reason: The market asks for it(and is also driven to). We get to keep in mind that there's always are more than one solution to problems such ISO performance and DR. To make the sensor bigger is one, but is it justified? This, I just can't know, or presume.
I Hope many of you read this to the end.
