It would be wise for some of them to try and distance outright from the name/brand, especially with branding (Looking at you dorb). That said, could they really survive?
Imakuni's health issues ensure she'll always be sporadic with streams. Jen disappeared willingly. Rev is Rev? No love for a commie. Potato is there, I guess? Never watched him.
Limealicious I think is the only one I think that realistically has any room to breath, having bailed to the UK.
Fred and Darren (Potato) are dead on arrival if they break off from Vinesauce. Neither of them have the viewer count or presence to completely separate from Vinesauce; the only way either of them get more people looking at their channels is people clicking through on Twitch wondering who the other Vinesauce members are. This would also be true of Jen if she hadn't voluntarily stopped streaming.
Direboar actually has a surprisingly sizable following - the one stream I caught of his recently (Mario Party 2) had somewhere around 250 people watching. However, I don't think he has any growth potential; he's not a very interesting person, and if anything, the only thing I found funny about the stream was how bad he was at the game. But I could just watch Vinny himself if I wanted to see that. I think cutting himself off from Vinesauce, at this point in time at least, would be a mistake.
You're dead on with Imakuni; I think she has a very niche, but ultimately dedicated fanbase, and one that WOULD support her if she decided to go completely solo, but she just can't. She'd never be able to maintain the streaming schedule one would need to do it completely full-time.
Joel is functionally independent in every way except branding already, and while he definitely COULD go solo, I actually think it benefits him more to stay on. So many Vinesauce fans now are teenage girls with unhealthy attachments to gamer men, and even though Joel and Vinny rarely interact, their being associated by the Vinesauce name keeps fans interested and lets them dangle the carrot of crossover streams whenever they please. Ultimately, he could really go either way, but I think it's both easier and smarter for him to stick around.
The only two people I can see gaining any benefit from disassociating with Vinesauce are Rev and Limes. Both have a big enough following to support themselves (for the most part), and a field in which they have little actual competition (Rev with the Flash games and stuff, and Limes with... whatever she does). Limes and Rev also have the benefit of not being associated with other Vinesauce members that often (though Rev crops up WAY more than Limes) and friendly relationships with other, non-Vinesauce streamers, which is something that Vinny and Joel both have been sort of slow to the draw on. They could break off if they wanted to, and it would probably allow them to expand their circle even more.
I guess the real question for them isn't "why haven't you done this," but more "do you even want to?" Because if I was any of these people, I can't imagine being associated with Vinesauce would be so confining and/or detrimental to my career that I'd care enough to separate myself. It's sort of a non-issue.