I too would like an option to export to the main 2 frameworks such as Foundation or Bootstrap.
My clients want a solution that includes a framework which makes it easy for them to change in the future.
Although Webflow is superb in designing pages, my clients (and myself) don’t want to be dependant on a particular platform. I know this means that some people may cannibalize the Webflow platform to create a website and then use their own hosting services and you ultimately would want people to use your platform and pay for it.
Perhaps you could create some tutorials on how to convert the Webflow source code into one of the frameworks. This way, people who really need it, will spend the time to do it, assuming it is a straightforward thing.