I wouldn't say that it doesn't work at all. The big justification I've always heard is that the wall will stop (or generally stop) border crossers after Trump is gone - even if a Democrat (or an overly business-friendly Republican) gets elected. I don't think that's true, because the wall's effectiveness is dependent on border enforcement. Without that, it's just a wall that can be pretty easily scaled. By itself, it'll stop quadriplegics and 90 year old ladies, but that's about it.
Suppose Biden wins in 2020. I think we know what he'll do with border security. It effectively won't exist. Well, if it doesn't exist, the wall won't be much of a factor. I'll refer back to the Berlin Wall. When there were thousands of armed East German guards with shoot-to-kill orders, it was tough to cross. Once those guards stood down in 1989, thousands simply climbed over. I don't see much reason to believe that won't happen with the border wall next time a Democrat occupies the White House.