It's not signing a rider who probably won't produce anywhere close to what he did in the past that's the problem, IMO. It's paying him according to what he did in the past. Sure, I can see signing Froome for just the publicity value alone, but how much is that publicity worth? Not the same as any rider who was perennially a very strong favorite to win the TDF would command.
Think back to 2017, when Froome won his last TDF, and added the Vuelta. If Contador hadn't decided to retire then, would a team have paid him what Froome was making at that time, on the grounds that he would bring so much publicity to them, and help attract other riders? Of course not. There's no conceivable math by which a rider who was once worth 5 million or so a year, but only several years in the past, is worth as much as a rider who is currently worth 5 million a year. You can argue that his past makes him worth more than another rider at the same current level, but without that past. So Contador in 2017 could have been worth more than another rider at the same level, because of what his fame and his history adds. But that isn't what Israel is paying. They're paying for Froome as he was several years ago, and even worse, paying for several years, in which he can only decline further.