I don't see the no-extra girls struggling. I do think the girls with limited menu's do make a little less than the girls who do more. In terms of them being better off w/o the competition from the extras girls, I think there is some merit to them making money on scraps from the extras girls, enough to outweigh any loss to them.
The way I look at it is, the majority of customers come in looking for lapdances that include some boob & ass grabbing. Those customers feel like they got a bonus if they got to suck a tittie, grab a monkey, or had a dancer grab their junk. As long as they get that level of service, they're happy and will keep coming back. They spend on the extra/non-extra girls alike, but the extra girls may sell an extra dancer or two due to aforementioned little bonuses. Then, there's the minority that come in looking for the whole enchilada. They spend on both girls, but quickly realize a bad investment and cut off the ones who show no potential of going forward. The money spent on them is what I'm calling scraps above. Those customers wouldn't be there without the extra girls, they'd visit massage parlors, escorts, etc.
I'd guess the scraps the limited menu girls make outweigh their losses to the full menu girls. So, the full menu girls are still making more, giving the perception that they're "hogging" all the money. However, the reality (in my very unscientific estimation) is those girls brought in that money and the limited menu girls are getting partial benefit of it. The net is, the limited menu girls are benefiting, not suffering from the availability of extras.