Two things. First, the DL shuttle is necessary because they have many flights checked in at T4 but still leaving from T2 gates. Second, I think the problem is more that T1 is for four flights a day, and third, the likely airline having to pay for that would probably be AF rather than DL (if I understand the BA and QF models I give as examples above and because it is mostly to allow AF to offer seemless connections, while for DL this is only one destination among many others) and I doubt that AF would like to do that.