It's actually not that difficult to roll your own WordPress site from an export--I was able to put together a site using exported Webflow code, the Genesis framework(with child theme) and Advanced Custom Fields to handle non-loop content--although I've also played around with what a UI for incorporating WordPress into Webflow would look like, because I'm a nerd like that
A custom CMS would solve a few problems, but the one thing they won't be able to leverage is something WordPress has spent 10 years building: a huge global community that has developed thousands of plugins. If I want to use ecommerce, I can choose from four or five options. Same thing with membership sites. I can use a plugin to split test everything from themes to page titles.
That's also why it would be so bloody hard to create a WP to Webflow port--the options are infinite and constantly evolving. How do you manage anything outside of core when the functionality is created by hundreds of open-source contributors, plugin authors and theme developers? It would be a massive undertaking.
For me, I'm probably going to continue using Webflow to build custom sites for clients and manage the WordPress conversion manually--sort of like a PSD-to-WP service, but I get to start with valid HTML, CSS and JS