Very briefly. I see what you mean. My thinking of this is a tad more abstract. I see SB, Django, Laravel, RoR, etc as more of a set of tools to achieve something. If your company would want to move to SB, would you go and hire a SB engineer? I assume you would not, but rather use the engineers you have and adopt SB on the go. Now, if you want to look at these still as skills, you technically could call them contextual skills for which you'll rely on foundational skills to adopt. A bit like a carpenter who knows how to use a manual saw, will quickly adopt a circular, water jet or laser saw.