It’s not even just that - you start taxing people who aren’t actually rich, you destroy their entrepreneurial spirit. You have a similar effect to the doctors who won’t work overtime because it negatively affects their pensions.
You put a brake on people working - why would you push yourself to work and earn more. People who earn £100-125k already have a marginal tax rate of 60%. Why bother, and just keep your income below £100k, add more tax on top of this and there is no incentive to earn more, and HMRCs tax take is overall lower.