Completely understand your concern. However, a couple of points to clarify:
- Webflow is meant for production site implementation, so your actual "source" files would be on your computer (in Photoshop, etc). You'd still need to use PS to slice images, optimize them, etc - we're not trying to replace Photoshop wholesale, just improve the workflow around the actual creation of the live site (something for which Photoshop was never intended).
- All your work completed in Webflow (to implement the initial design, and make it responsive), can be easily exported to a full HTML/CSS/JS bundle that can be run hosted anywhere you wish. A good practice could be to download completed sites and archive them in source control as a contingency.