I think the 8% would be something like the effective tax rate they paid over all their earnings, by using whatever tax loopholes their accountants come up with.
To me that is part of the problem: those with money can pay expensive accountants to come up with schemes to reduce tax bills that are simply not available to Joe Public. So while JP pays c.20-30% on their wages, or whatever it is, the wealthy find ways to pay single-digit %s.
Couple this with the disproportionate focus that, at least in the UK, the inland-revenue give to chasing and penalising non-payment by Joe Public, while making deals with the wealthy that still seems them ultimately pay disproportionately less %.
I get that wealth makes wealth, and I generally don't begrudge people making vast amounts (esp. if I see what they do as benefitting society), but I am a socialist to the degree that I'd like to see a fairer tax system, not just in higher earners paying.an increasingly higher tax %, but actually more in seeing an end to loopholes and schemes that really only benefit the wealthy. Schemes that give tax breaks for benefitting society, sure, not an issue (in general) but I'm more talking the straight tax avoidance for the sake of improving their own net income.
The UK has been lax at closing tax loopholes, loopholes that are, on the whole, not available to Joe Public because they can't afford the people who set them up, or advise on them. So they benefit the high earners disproportionately, resulting in higher earners not paying their dues while Joe Public pays a higher %.
Personally I think it's a result of a tax system that's far too complex, inviting the loopholes. But that's maybe an entirely different thread.
Then there's a question of who exactly these higher earners are, and what they're doing to earn their money. The only issue I have with these would be those that are given huge bonuses based on short termism that can directly affect the public, when longer term performance should be the incentive.
E.g. a.fund manager whose fund grows 50% one year, then loses 66% the next... that manager might get a massive bonus one year, but not have any real penalty then next, while Joe Public who invests in his fund suffers.
Sure, simplistic, but hopefully gets my point across.
On the whole, I can see the benefit of tax cuts, but I'm certainly one for fairness as to who they're aimed at. The system isn't fair in most countries as it benefits those who can afford to make use of loopholes. But as an economic strategy, sure, tax cuts can benefit all, and the wealthy will, even without such loopholes, likely benefit more.
The problem is that no legislative body enacted something called a "loophole". It's a "loophole" either when you don't understand it or when it doesn't apply to you because it's not useful to you.
There are business or economic reasons for any tax reduction. You can agree with or disagree with the reasoning but it's there.
In the US. you would have a personal exemption on your tax form, you would be able to deduct mortgage interest payments, if you sell your house you don't pay tax on the first $250k of capital gains and if you are married you exclude $500k in capital gains.
You have a 401k at work that isn't taxable and a Roth IRA that isn't taxed when you take the funds out. You also have exemptions on your tax return for dependents and if you are lower income you would have child care tax credits.
You could shelter a good deal of your income if you wanted to. Is that "fair"? Most of the services go to "you" and you pay for very little of it via taxation.
You have a job because someone is investing in the economy. You can't like a growing economy but then decide that you don't like the tax provisions that enable that growing economy. It doesn't work that way.
The services largely go to the lower and middle class and not to the rich and yet they pay millions of dollars in taxes but some don't like the percentage they pay. They are still just one person. A very productive person. The middle class don't want to pay any more for anything.
They also pay taxes that don't get talked about. They, potentially, have payroll taxes, property taxes, their part of social security taxes for employees and the list goes on. No one talks about that when they are worried about "fairness". Biden wants to greatly increase corporate taxes all the while pledging to not increase any taxes on any individual who makes up to $400k a year. $400k!
Is any of that "fair"? Is anyone really trying to be "fair" here? Do most people who want the rich to pay their "fair" share even know enough about the subject to know if the rich aren't paying their "fair" share?
The laws that get enacted as a result of this sort of "outcry" generally don't have much impact on the rich and it just increases certain taxes on the more productive members of the middle class. The ones who invest. The ones who live where $250k doesn't cover their home equity. The ones who at retirement might sell assets for retirement and now have higher taxes that were intended to make the "rich" pay more.
It doesn't matter how much someone's salary is, how much their bonus is. Government isn't in the business of micro-managing businesses, telling them how much to pay people, how much their bonus should be, how much a credit card late fee can be, etc. That's not their job and they don't have the knowledge, expertise and data to be micro-managing a company like that. It only leads to bad (and stupid) outcomes.
The government wanted to "help" those who couldn't afford a house to buy houses in the early 2000's. They didn't give them more money, they just convinced banks to write sub-prime mortgages. How did that work out? It didn't. It almost brought the entire financial system down.
Then they gave loans to students so that everyone could go to college but many didn't graduate and many incurred more debt than their degree could ever earn. Now the government is having to "forgive" those loans. It's entirely predictable and stupid.
So, why pass tax law to encourage investing and then turn around when it's politically expedient to "tax the rich" more on those some laws? Again, it's stupid to both encourage and discourage the same act.
The only reason people are saying that the rich don't pay their "fair share" is because that narrative is constantly repeated in the media and with articles that appear to show some correlation but no causation. "Look , house prices are high and the rich are doing well, that's not fair, they must not be paying their fair share!" Keep repeating this and suddenly it must be a fact.
Nothing good comes from populism whether it's on the right or on the left. Joe Public, as you put it, isn't particularly well informed on how the system works and making decisions based on his emotions isn't good, long-term public policy. There is a case to be made for a term limited benevolent "dictator" (with appropriate laws limiting personally benefiting from such an arrangement).
I'm not actually pushing for that, especially in the age of Trump, but it works in companies, the military and in certain unruly countries (to a degree)

What we have now is just a clown show. The US is the obvious example but the UK is not doing well economically and Brexit was stupid and a result of populism.