D-ticket was well intentioned but a bad idea from the start. Discalimer: not a German and I only used it once (well twice because it was impossible to buy just one month)

The administrative overhead of centrally gathering all the funds and then portioning them to individual agencies must be hell. And according to what criteria? It makes the responsibility for funding even more murky than it is. Regional tickets also have to keep existing so it means the entire old ticketing system must remain functional even though few people are using it.

Germany led the way on tariff integration with it’s “Verkehrsverbund”. The difference being that service planning and ticketing were done by the same agency. Involving the central government doesn’t seem like a good idea, sooner or later national politics will start influencing local transit.