The conclusion I'm leaning towards is that the Saas version should, on top of the hosting/maintenance, offer some distinctive features ( it depends of your product, but on an Saas model, your architecture can be more complex than a Backend/Frontend/DB architecture, thus allowing more advanced features ).
In my view there is three layers, that could be seen as three steps ( but maybe I'm wrong ) : open source ( great for WOM if well managed, great to harden the software ), enterprise-behind-the-firewall license to make recurring revenue and build a sales pipeline, Saas once the architecture is stable enough to handle hundreds/thousands of customers with acceptable costs of operations.
Someone can take your code and set up in competition to you using the OSS version, just changing the logo. But the software is at the end only 30% of a company. What makes a great company is how it evolves with its market to serve and nurture better its customers. Economy is not about selling products, it's about exchange between humans. The real product is in the experience ( the software, the tone, the customer service, the email, the documentation, the ads... every little byte, pixel, sound that your company emits ). A company that steals your OSS and monetize on it without making it a new experience is just like a guy around the corner selling Rolex replicas.