How is it not already? Because it can't do layers? Or pixel-level editing?
My one major grief is that you cannot extend the canvas, i.e. on rotate there's no way to heal-fill an area, you *have* to delete the part that is rotated out of the rectangle. The same goes for fixing a too tight framing. This is stupid because after a trip to a pixel editing app (converting raw->tiff, doh) and adding some space around the shot LR's heal/clone brush works just fine for this purpose.
Sure, but if we took a poll I'd bet we get a long list of wants that could be put into Lightroom. Put all those together, and you have Photoshop.
LR was designed as a database management program, along with the tools to non-destructively "develop" your photos, in a traditional sense - exposure, colors, gradients, white balance, sharpening, cropping, B&W, etc. They've gone beyond that in adding things like the new healing tool and localized adjustment because it's so frequently used by photographers. Don't get me wrong, I think it's great they put that feature in, one of the best additions in awhile. But it blurs the line between non-destructive LR and pixel-level PS. And you have to draw a line in the sand somewhere. I get what you're saying, you could use the healing tool that's already there to draw in the boarders, but at that point you're creating pixels, and I just see that as PS territory. Not to mention, I can only imagine what it would require to do that non-destructively.
They're welcome to put all the features they can in LR (providing it doesn't slow it more). I'm certainly not against that. But I don't see criticizing it as being a lousy program for doing something it was never designed to do.