TL;DR: Allow devs to upload a second version of their template to the marketplace—one that follows Client-First naming conventions, if they so choose.
I've completely adopted Client-First by Finsweet in my work. I've worked with others, like SystemFlow, but CF made the most sense and seemed the most efficient solution.
A client of mine purchased a template from the Webflow Marketplace, and I started using it for them. The class names seemed similar to CF, but they weren't formatted in the same way, so I noticed and moved on. Later, I found the Style Guide in the Designer, and lo and behold, they mention the template is based on Client-First.
The issue is that their effort to be modeled after CF is largely in vain once they've had to go through the template submission process and completely change the class name formatting.
Furthermore, Webflow's naming convention decisions and standards make zero sense to anybody who actually coded before coming to Webflow. I understand you're trying to make it simple and are all about no-code, but I believe there's value in remaining as faithful as possible to the underlying languages that Webflow is built on. Going lowercase and using hyphens is not a super scary thing, nor is it actually confusing. People grow accustomed to whatever they end up seeing.
But… as that's clearly not the direction you guys chose to go, I'd love to see an option for Webflow template developers to upload two versions of their template—one to satisfy your standards and naming conventions, and another in the form of Client-First (if that dev wants to do that). As it stands, there are already third-party solutions out there helping devs convert their templates from CF to Webflow's naming conventions, so I'm clearly not the only person concerned about this.