I think that's a specious argument. Can we say that Bernie Sanders is "not serious" about his proposals because he provides no detail for how to implement them beyond a repeated bellow that things aren't fair as they are?
No, the reason he can't be dismissed just because he has no blueprint for how to finance his "plan" for revolution is because he has a moral imperative - people know that the things he says (although often admittedly not much more substantial than platitudes) about income inequality and international trade are true.
NO -it's understandable that everyone disagrees about how Bernie's proposals might be made feasible. It's totally understandable and credible to say that the calculations necessary to bring Bernie's plans to fruition will be difficult to say the least
.It will involve navigating clashing economic and sociological theories and political agendas to even begin planning how to proceed, if Bernie becomes president.
Similarly, it is ridiculous to say that anyone advocating for serious consideration - consideration, mind you - of reparations is "not serious" if they are speaking without an economically feasible plan already in hand. It will obviously be an extremely difficult plan to make - fraught with emotional landmines and clashing economic and sociological theories.