I doubt very much that retention would be seriously hurt by better GI Bill benefits.
More schools are going online with their degree programs and often have discounts or scholarships for Active Duty servicemembers. A school I work with (not for) has a 10% discount for AD personnel; 5% for vets. They've got an entire department nearly 100 strong dedicated to serving Active Duty military students. (They also recently awarded 25 full scholarships--tuition, course materials, and Apple laptops--to wounded warriors at Walter Reed; but that's beside the point.) Schools are going after AD troopers, not just veterans.
The GI Bill benefits can be used by many of the AD personnel. Sure, a good portion aren't going to want to do school while they're fighting a war; but many do. There is a lot of incentive in the Corps to get a degree while still on Active Duty. It looks good on your reviews, and you can't get a commission without it.
Besides that, tuition assitance for Active Duty Marines is more than just the GI Bill. If you can take the time to go to school while you're in the Corps, it's much cheaper to do that than to get out and go to school; and you might see some additional coin later on down the road after you've got your degree. I got my degree after the Corps, and my employer didn't bat an eye. There was no raise. I wasn't given higher scores on my review or considered for a promotion.
I'm not saying that there wouldn't be a few more people who get out if they had better education benefits; but in my experience, people who were in the Corps didn't sign up for a college education. That may be different in other services, but I don't think that its as big a problem as they're letting on. (Where the money is going to come from is more of a concern for me.)