The purpose of progressive taxation is to put the greatest burden on those with the means pay more. This makes economic sense due to the theory of the marginal utility of money: a further pound to someone on £100k is worth less than a further pound to someone who earns the median wage of £26,000 or so. means you pay a greater proportion of the 100,000th pound than you do of the first. And for a quarter of a century, the British income tax rates were more or less fixed, apart from an abortive experiment with the 10p starting rate. The rich paid at a marginal 40% and everyone else paid at a reasonable 20%.
Ultimately the mechanism by which the right-hand side of the Laffer curve works is by shrinking the pie, not just for the rich who pay the taxes, but for everyone.