My wife runs her 2-person agency on Webflow. I’ve seen her go through this same situation.
She’ll either have clients:
- too scared to use Webflow and stay on their own platform (usually Wordpress)
- loves Webflow and lets her keep the site on her account while giving them CMS access
- loves Webflow but would rather keep the project on their own account
My suggestion to you is if the client wants to create their own Webflow account and wants the finished project transferred to it, then do it. If they break their site design or need additional help, just create a new contract to help them fix it.
Just be flexible and do as best of a job as you can for your clients and they will always come back to you or refer you to their friends. 
good luck out there