It depends on who is the owner of the fiber.
The incumbent carrier typically has enough fiber strands to avoid any colored interfaces (that are 3x expensive compare to gray) in the Metro.
Metro ring typically has 8-10 nodes (or similar). 16-20 strands of fiber were not possible to construct anyway – any cable is bigger.
It is the same cost to lay down fiber on 16 strands or 32.
Hence, PTT just does not need DWDM in Metro, not at all. Hence, the DWDM optimization that you are talking about below is not needed too.
If you rent a single pair of fiber then you need colored interfaces to multiplex 8-10 nodes into 1 pair on the ring.
Then the movement of transponders from DWDM into the router would eliminate 2 gray interfaces on every node (4 per link): one on the router side, and another on the DWDM side.
Overall, it is about a 25% cost cut of the whole “router+DWDM”.
It is still 2x more expensive compare to using additional fiber strands on YOUR fiber.
By the way, about “well-defined stack of technologies”:
NMS (polished by SDN our days) should be cross-layer: it should manage at the same time: ROADM/OADM in DWDM and colored laser in Router.
It is a vendor lock up to now (no multi-vendor). Hence, 25% cost savings would go to the vendor that has such NMS, not to the carrier.
Technology still does not make sense because no multivendor support between the NMS of one vendor and the router or DWDM of another.
Looking at the NMS history, it would probably never be multi-vendor. For that reason, I am pessimistic about the future of the colored interfaces in routers (and alien lambdas in DWDM). Despite a potential 25% cost advantage in eliminating gray interfaces.
PS: “routed optical networks” is proprietary marketing. Nobody understands what you mean. I did google to understand.