Well, that’s branding, isn’t it? People would say “I’m playing Critical Role tonight!” and folks would know what that means. Building a brand is hard, but if anybody in the TRPG space, CR would have the best shot at it.
I agree. If anybody has a shot of creating a successful RPG based on branding without any real game design experience (not to discount Mercer's ability and homebrewing), it would be them.
That said, I don't know if it would be as lucrative as what they do now. You of all people should know how much work and effort goes into creating, designing, and publishing an RPG. These people are mostly just actors, and a lot of them have a minimal interest in even mastering the system they are currently using. They were in it for the characters and the story-telling, and largely rely on someone else to tell them how a specific ability, spell, or feature might work.
And this may even extend to how their viewers feel about watching them. How many viewers check out a couple episodes because it is D&D, and how many
subscribe to the show because of what they see? I know personally, I find their personalities, their story telling, and their personal interactions to be far more interesting than the actual dice rolling and combats.
In fact, I think there are better systems out there that would be a better fit for their style of play and narrative improvisation (like most Fantasy Flight Games RPGs, for example). It may have been the Dungeons & Dragons brand that put them on the map. Well now they are on the map with a brand name of their own! Imagine how much another game company with its own RPG would benefit if they sponsored a new Critical Role series using their game system? And judging by the numbers you listed in the post, I think that would be a LOT more lucrative to them than to publish another RPG system trying to compete in such a small market already dominated by competitors who have a lot more resources, experience, and their own brand recognition as game designers.
Not saying they can't, won't, or shouldn't. I just think there are much better options available to them, and at some point they will realize that they could start to consider divorcing themselves from D&D/WotC and create more opportunities for themselves.