Taxes are not there to provide incentives, but public policy does act as an incentive or disincentive for people's actions, especially at a monetary level.

When you say wealth should theoretically come from past taxed income, you open yourself to the retort that wealth does not, in practice, come from there. We're not designing some sort of utopia here, but talking about pubic policy for the here and now. And, in fact, John Stuart Mill (again) argued

The principle of private property has never yet had a fair trial in any country; and less so, perhaps, in this country than in some others. The social arrangements of modern Europe commenced from a distribution of property which was the result, not of just partition, or acquisition by industry, but of conquest and violence: and notwithstanding what industry has been doing for many centuries to modify the work of force, the system still retains many and large traces of its origin. The laws of property have never yet conformed to the principles on which the justification of private property rests. They have made property of things which never ought to be property, and absolute property where only a qualified property ought to exist. They have not held the balance fairly between human beings, but have heaped impediments upon some, to give advantage to others; they have purposely fostered inequalities, and prevented all from starting fair in the race. That all should indeed start on perfectly equal terms, is inconsistent with any law of private property: but if as much pains as has been taken to aggravate the inequality of chances arising from the natural working of the principle, had been taken to temper that inequality be every means not subversive to the principle itself; if the tendency of legislation had been to favour the diffusion, instead of the concentration, of wealth--to encourage the subdivision of the large masses, instead of striving to keep them together; the principle of individual property would have been found to have no necessary connexion with the physical and social evils almost all Socialist writers assume to be inseparable from it.

None of what JS Mill said in this paragraph is significantly less true today than it was 150 years ago. And, in fact, looking at statistics of wealth and income inequality things have not been getting any better for the past 30+ years.

As to "any tax is expropriation"

the Distribution of Wealth [is a] matter of human institution solely. The things once there, mankind, individually or collectively, can do with them as they like. They can place them at the disposal of whomsoever they please, and on whatever terms. Further, in the social state, in every state except total solitude, any disposal whatever of them can only take place by the consent of society, or rather of those who dispose of its active force. Even what a person has produced by his individual toil unaided by any one, he cannot keep, unless by the permission of society. Not only can society take it from him, but individuals could and would take it from him, if society only remained passive; if it did not either interfere en masse, or employ and pay people for the purpose of preventing him from being disturbed in the possession. The distribution of wealth, therefore, depends on the laws and customs of society. The rules by which it is determined, are what the opinions and feelings of the ruling portion of the community make them, and are very different in different ages and countries, and might be still more different, if mankind so chose.

The very concept of expropriation is a human convention, as is property itself, and Western™ ideas about property can be traced back to Roman Law and the idea of absolute right to dispose of one's property.

tens of millions of people stand to see their lives ruined because the bureaucrats at the ECB don't understand introductory economics -- Dean Baker

I think that taxation of earned income should be relatively low, but taxation on unearned income should make up for it.

It is not the owner of a productive asset who earns money, but rather the asset, and there is no reason whatever why privileged property rights - such as exclusive rights over the commons of land or knowledge; or 'free' limited liability on investment through joint stock limited liability corporations - should not be taxed.

It is a misleading canard to say that people are taxed twice: they are taxed once (too highly IMHO) on earned income and then the income from the asset is taxed.

There is a bonus here in that taxation of such property rights is much less avoidable, and much easier to collect. Friedman himself considered Land Value tax to be the 'least worst' and most economically efficient tax.

So I would (if I did not regard taxation as pretty Last Century)  advocate drastic cuts in taxation of earned income and commensurate increase of taxes on unearned income from privileged property rights.

"The future is already here -- it's just not very evenly distributed" William Gibson

Taxation of unearned income or wealth is not expropriation, is the point.

tens of millions of people stand to see their lives ruined because the bureaucrats at the ECB don't understand introductory economics -- Dean Baker

Aren't the poor more likely to be renting? Are you advocating rent control as well?