Physically, I don't think she would have any problems. Since the advent of power steering, Cup cars don't take that much brute strength to drive anymore. If you'll check the stats, most of the guys coming into the sport now are rather small compared to the six-foot, 200-pounders of the past. Mark Martin ain't a whole lot bigger than Danica.
The competition? It's brutal, all the way through the field, compared to open-wheelers. Can she handle it? We won't know until she tries.
Will she get treated rudely by her competitors on the track? Actually, that depends on her. The example people often give is Scott Pruett, how he got wrecked just about every week the year he tried Cup full time. Some say it's because he was an Indy-car foreigner infringing on NASCAR's territory. My understanding is that his behavior and attitude toward his peers contributed mightily to that.
Bottom line, IMHO she's welcome to try, and whatever happens, happens. Quite frankly, though, I'm not all that sold on her driving ability. Her nearly winning at Indy wasn't exactly a fluke circumstance, but it had more to do with a bunch of other factors including good pit crew work than with her being dominant on the track that day. I will say in her defense that this year, Rahal's cars are pretty much crap, so I don't know if anyone else could be doing any better.
I actually think Montoya has a better chance of not stinking up the joint and being reasonably competitive the second time around to all the tracks than she would.
Think you nailed it, GB...what are they afraid of?