Taxes on the rich clearly aren’t too high
There are lots of countries in the world that are tax havens. They are short of skilled labour. Anyone earning enough to pay higher-rate tax in a Western country has a skillset that would easily land them a job doing something similar in a tax haven.
Instead, they’ve chosen to live where they do. Definitionally, this shows that they believe the tax is a price worth paying for the quality of life they enjoy there. If they didn’t, then they’d have moved to a tax haven already [*].
So while some rich people might complain that they think taxes are too high, they clearly mean this in a “it’d be nice if this thing was cheaper, but I’m still going to buy it at the price it’s on sale for” way (rather like people buying Apple products), and therefore we can discount their protests.
Taxes on unskilled workers, who don’t have the same advantages when it comes to free migration, are a different story: the poor can’t be deemed to have agreed to the deal in the way that the rich clearly can.
So the morally best way to reform the tax system would be to remove the working poor from the tax net, while ensuring that those wealthy enough to have a choice bear more of the cost. This is even before you consider the massive benefits (on virtually all measures) of having a more equal society.
[*] there is a pragmatic argument that “we’ll be stuffed if all the talented people leave”, and there is presumably a level of tax at which this might be true. However, evidence from the 1960s and 1970s (when marginal tax rates on very high incomes were above 90% in the UK) suggests that the proportion of talented people leaving even at that rate was low enough as to be irrelevant to overall economic growth.