Jeremy Cobyn’s proposal to bring in a maximum wage (or, if you like, a 100% tax rate) would not work, for two reasons. The first, is that the swing voters he needs to attract would never vote for a party with this potty policy. If you are flirting with voting for the Conservatives, you are not going to like the idea of a wage cap.
If any further discussion is needed, then the second reason is that it would not have the intended effect. Presumably the reason to bring down wages would be to reduce inequality. But most rich people do not get their income from salaries, but from dividends, capital gains, rental income etc.
I don’t know if the Duke of Westminster takes a salary, but capping it would not change one iota the fact that he owns a huge swathe of London’s most valuable real estate. If your only source of income is your salary, then capping it only stops you being able to catch up with the already-rich. Or encourages you to take income in other forms.