I really don't see what is wrong with the 50p tax rate, in fact, I'd like to see it extended to capital gains of £150,000 too. Lots of successful European countries have 'super tax' for the mega wealthy.
I also doubt that it isn't raising money either, if it wasn't then that might be another matter. If it is not raising money then it's more likely to be that the mega wealthy are dodging/avoiding tax rather than simply not working because of the 50p rate. In which case the solution would be to clamp down hard on tax avoiders, every bit as hard as we clamp down on benefit cheats and stigmatise it in the same way.
The thing is though, the Tories actually don't mind tax avoidance. Labour were pro tax avoidance too and left special non-dom loopholes open to be used by Russian billionaires to enable them to come and live here and create a property bubble in central London. I'd hate to think how much mafia money has been laundered via property in central London. The Lib Dems on the other hand don't actually have any principles at all, they're just saying this because it is popular, they could do a u-turn on it tomorrow if the political winds changed.