Those bans are being challenged in court, and I doubt they will hold up. It would be impossible to differentiate between a delivery truck with ads on it and a truck with no cargo. It also seems unlikely that the cities will want to lose their transit income, which are ads they place themselves, since a court will probably view that as mobile advertising, as well. Until it is decided in court, I would not write it off. Los Angeles has been trying to ban billboards for years, and they never hold up in court.
If they do succesfully ban mobile billboards, that just makes regular billboards more valuable.