I'd say it could be done, and has, but it's a big hill to climb. If you recall there have been various acts as you've mentioned in the past, Anthrax/Public Enemy, Faith No More, Cypress Hill, kind of the Beastie Boys. I'd say there is less competition maybe, but regardless, what you do needs to be top notch and good enough to make it big. Same as all the other genres. It doesn't matter if it's Gospel/Rap, Country/Techno, or Rockabilliy/Classical if the song has all the magical components falling into place, it can make it big. It may be harder to go it alone, and especially when costs/finances are concerned. I will say though, if that's what you want to do, then go for it! Start as one and add other musicians as needed and when you can. Of course going it alone does depend on your musical abilities/ear too, not just the vocals.