The six year limit only applies to the prior service members that cannot qualify to come in as an E-4 or higher. For instance, if they had a position that was on the open rate list, and a rate determination package was approved to bring them as an E-4, they could have more than 6 years. It all depends on their qualifications and the determination of the rating force Master Chief for that respective rating.

To answer your other point, in the current climate, I wouldn't expect someone to be able to continue past their first enlistment without having made E-4. If you were on a four year contract and you were still waiting on a long 'A' school list, that could be forgiven, but there would be no reason to not make E-4 or above in 6 years. I made E-6 before my 6 year mark.