Who is "we"? DL is doing just fine and while that does not mean that consumers absolutely accept the C+ allocation, it is a sign that overall, consumers are purchasing DL seats at DL fares.
The problem here is that OP wants C+ for free and expects DL to expand its C+ "cabin" accordingly. That is highly unlikely to happen. If there were a market for C+, e.g., a true premium economy product, DL would likely move that direction on its standard domestic routes. But, as a giveaway of a product which has the built-in problem of taking more space, that is dreaming.