I would love for this hypothetical tool to exist, but the problem doesn't lie with the tool, but that you're talking about essentially different workflows. Designers tend to use the tools that they're most familiar with, that give them what they need. Usually that means Illustrator + Photoshop (or sometimes, Fireworks).
I've given thought in the past to a hypothetical tool that I'd love to see that does a lot of what Photoshop does. It would use a nested plaintext file structure, letting it work with a version control tool (imagine being able to diff two PSDs and know that all that changed was a label value, or a single layer being removed, rather than it being a completely opaque binary change?). It would effectively provide direct access to the advanced features of CSS, only with a nicer UI. It could be built to allow testing responsive layouts all within a single document. All this and more. It'd be great.
But designers wouldn't want to use it. It wouldn't be grounded in their essentially visual workflow. Sure, you could have intermediaries translate documents between the visual and the quasi-layout-based, but that's even more work than just having designers adopt better practices. What's more, these practices don't just benefit developers: it'll make life easier for designers too! That's what makes it such a no-brainer. Sure, some lazy or precious designers might object to being told how to do their job (nobody appreciates being told they're doing things wrong), but if you introduce these practices gently I'm sure they'll pay off.
I've given thought in the past to a hypothetical tool that I'd love to see that does a lot of what Photoshop does. It would use a nested plaintext file structure, letting it work with a version control tool (imagine being able to diff two PSDs and know that all that changed was a label value, or a single layer being removed, rather than it being a completely opaque binary change?). It would effectively provide direct access to the advanced features of CSS, only with a nicer UI. It could be built to allow testing responsive layouts all within a single document. All this and more. It'd be great.
But designers wouldn't want to use it. It wouldn't be grounded in their essentially visual workflow. Sure, you could have intermediaries translate documents between the visual and the quasi-layout-based, but that's even more work than just having designers adopt better practices. What's more, these practices don't just benefit developers: it'll make life easier for designers too! That's what makes it such a no-brainer. Sure, some lazy or precious designers might object to being told how to do their job (nobody appreciates being told they're doing things wrong), but if you introduce these practices gently I'm sure they'll pay off.