Brownlee could do it for sure. Everyone is mentioning that he'd have to learn how to ride his bike in the bunch. He has some of the best handling skills in draft-legal triathlon. Read: He is already great and handling his bike in a bunch. He trains almost exclusively with cycling road racers and has since he was 12 or 13. His TT times easily put him into continental level racing. He might just be a work horse, but he could ride professionally fairly easily. He wouldn't get paid much, but he'd get paid.
As for running....
He (Brownlee) races cross country in the off season. My old roommate was a 3x top 20 at NCAA and he races neck and neck with Alistar on a pretty regular basis (he's English). This is with swimming in the mix for Alistar. My ex-roommate just ran for a few years. He didn't make much money, but he did get paid.
I think you'd be surprised how many of the top level triathletes could race domestic pro on the road. I'd say 10-15% could do it.
Running, Probably 5-10% of the ITU athletes could pull it off and almost none of the non-draft pro triathletes could pull it off.
I've coached full time pros in road cycling, Cyclocross, marathon and ultra running, but mostly triathletes. I've been around enough to know who gets paid what.
Just think, if you had those skills, wouldn't you rather swim a bit and make a healthy six figures in tri or just ride/run and barely get by with two different sports.