Hard drive space really isn't an issue. It's quick, simple and easy to just pick up an external hard drive and plug it in by USB. Problem solved. X360 titles were also smaller in size since they were only on regular DVD's. So they don't eat up as much space as the titles on the X1 and PS4.
For now, the list is very selective of games, in the beta test phase in the preview program. I've had no issues with the ones I'm playing.
I don't think the issue is people still WANT to play old games, it's the fact that you bought it. You still own it. You bought it digitally, and when you move to the X1, it's "gone". Come November, it won't be (hopefully as that list grows anyways). So, its the opportunity to still get some value out of something you own, without having to keep the console laying around.
Everyone knew it would be hard to get backwards compatibility on these new consoles. MS pulled it off via emulation. For Sony to sit back and go, "Well, if it was easy we'd do it" or something along those lines is basically saying, we'd rather charge you for it via PSN Now because we've invested too much into it. I wouldn't be surprised if they change their stance on that down the road.
I think this move should help a lot of the current 360 users migrate over to an X1. I think it will also help some of those who had 360's, and are fortunate enough to afford a PS4 and X1, but switched to PS4 at next gen launch, go out and pick up an X1 now too. Especially as that list of BC games continues to grow. You get the value, out of stuff you already own. It'll definitely help sales I'd bet. Whatever happens, I'm just a bit surprised at Sony's response.