From what I can tell that message is the fallback content. If features such as forms etc. aren’t supported then that message is displayed. If I understand correctly, what you want is for Sumatra to forcibly attempt to display as much of the document as possible, regardless of whether it involves unsupported features, right? If so, I don’t see how that would work. Take for example a document that uses scripting to dynamically display or hide additional content/fields based on user entry in a form. How should Sumatra handle this? Display all the hidden text sections/layers, even if they would overlap (since ordinarily only one among them is meant to be displayed in a particular location)?
PDF is a presentation format meant to display documents (including text, images etc.) in a platform-agnostic manner (i.e. independent of the viewing software, hardware, OS etc.) If a viewer doesn’t support certain features then not displaying the document at all is probably the right course of action, rather than choosing to display it partially in a manner that looks nothing like what the author intended.
Last but not least, from what I can tell a major change like this would need to be made in the underlying MuPDF engine and not in Sumatra itself, although kjk would know best if that’s true.