There are many not-so-great ways to go to CDG: RER, bus, taxi, TGV from province (even the TGV is not great because of the lack of frequency and difficult connections). The express link to Gare de l'Est also seems... not great. Who will it serve? People around Gare de l'Est generally cannot afford it, people in the center and West will generally be better off using other transportation modes.

Bad proposition.

A good proposition is to link La Defense, Etoile, Invalides, Opera and Gare du Nord to CDG 1, 2 and 3 with no connections. And ideally this service should have its own private underground track. Of course this project would fetch maybe 10 billion euros and thus would not be a great proposition... or maybe think twice...