> By "worked well" I mean people paid their taxes and the overwhelming majority were law abiding.
as i understand, the "overwhelming majority" (lower and middle class) are not the focus of the problem here as they rarely use tax havens to circumvent taxes anyway (it's probably not cost effective). this is all about those who have the funds and motives to circumvent taxation, who happen to be a minority - even thought they're not a minority when it comes to taxable income; the 1% or whatever.
as for the rest of your argument, i don't completely agree. while you're probably right in practice, i.e. the authorities would have used their tools against those movements, the goals of those movements were different. the civil rights movement was probably illegal at the time but the goal was to change the laws to make it legal. same for sex. no millionaire wants to make tax evasion legal, as that would affect all tax payers, even those of the lower and middle class. they want that only for themselves.
i agree that (hard) drugs are probably a comparable example - it's illegal and puts a strain on society. on the other hand, use of hard addicting drugs can be categorized as an illness. so should we label money hoarding and tax evasion an illness? offer treatment options? allow them supervised safe spaces to evade taxes so we can offer them help?
i'm not convinced that financial privacy whould be no exception. there must be limits if it affects the others. i also don't think environmental pollution should be an issue of privacy - you shouldn't be able to pollute the air and claim it's nobodys business, as your factory stands on your own private plot of land.