If AT&T had been smart, the unlimited data plan would be stuck at the network speed that was available when the plan was entered into. I don't see any reason why LTE speeds necessarily should be offered to unlimited plans that AT&T stopped offering while the network was only 3G.
Or simply eliminate grandfathering unlimited plans and giving users a choice to leave ATT or go to a capped plan. They do not have to continue to offer unlinited plans but have done so for business reasons. Get rid of them at the end of the contract date and the problem goes away; and I say this as someone with a grandfathered unlimited plan.