...
First of all, how does people BUYING THE UNTRANSLATED MANGA, harm the mangaka?
You don't exactly make a good case, for yourself, when you start of, on a foundation of completely misrepresenting what I said.
Dishonesty doesn't make you look good, or sound convincing.

Secondly, there is ZERO evidence, that pirate copying, has ever had any negative effect, on the income of any mangaka/artist/musician/author/whatever.
Not with anime/manga, movies, TV shows, music...
On the contrary!
Pirating music is know to have increased music sales, as people got exposed to more music, that they'd like!
Legal streaming, on the other hand, has had a massive negative impact, as artists only get a very tiny proportion of the income. This was true with vinyl and CD's too. Artists made almost no income, from CD sales (just living on concerts), with all the money going to the record label ...and they get a far smaller proportion, from streaming.
None of that, however, is the fault of customers. It's purely the fault of record labels and streaming companies.

...but to get back to manga!
I present these scenarios, of what happens if people read scans vs them not doing so, and ask how any of them, cause any harm, or lack of income, for the creator:
1. A person first reads scanlations of the manga, and then, at a later date, buys it.
VS
Buys the manga, and reads it.
2. A person first reads scanlations of the manga, and then, at a later date, buys it.
VS
doesn't buy it, because they don't know about it, or don't realise they'd like it.
3. A person first reads scanlations of the manga, and then doesn't buy it, because their budget won't allow it.
VS
Doesn't buy or read it, because their budget won't allow it. (if they're even aware of it, or realise they'd like it)

How does scans have any negative effect, for the mangaka, in any of those scenarios?
How does scans not benefit the mangaka, in scenario 2?

Nonsense!

Though if there is any tendency towards not buying manga, then it's because what is available to purchase, i.e. commercially "translated" manga, is utter shite.
Why would you want to give money, for an atrocious product?
Why support/encourage such abominable behaviour? (it would indicate a desire, for manga to continue being "translated", in such a fashion. For Western manga releases, to go further in that direction)

...and buying raw manga, isn't really something most would consider, as not only is it far from clear and simple to do, but most can't read any of it. Having a physical volume of incoherent gibberish, to accompany ones scanlations, feels a rather weird thing to do, for most people.

...but if they're unwilling to buy the commercial "translations", then the option would be for them to never read it, at all.
So either they don't buy it, or they don't buy it.
Either way, the mangaka gets the exact same amount!

Why should someone pay compensation, for a fundamentally broken product?
There is no justifiable reason, to pay such a "translation".
Appreciating the mangaka's work, and considering it worth paying money for, is not the same as appreciating the commercially "translated" release, or considering it worth paying a single penny, for that!

On the contrary, I wouldn't want a badly "translated" commercial release, even if you paid be to get it, and I consider it an affront to the mangaka and the original work! And also a grave insult, to any and all readers, as well as the concept of translation.
It's also fraud.

Oh? And how much do they know about the commercial "translations" of their works? About the quality of the translations? ...and how much do they know, about the consequences of scanlations?
Nothing.
They don't know anything, about any of it!

It's not their work. It's a pathetic misrepresentation, of their work!

Without scanlations, no one would know about small or unknown mangaka, and thus wouldn't buy any of their works.

No they don't.
The vast majority of their income, is from domestic sales.

...if you're okay with reading worthless trash.
Not only do a lot of the best stuff not get licensed, but the commercial "translations" of what does get licensed, is an abomination, that is quite different to what the mangaka actually wrote!

There's more than enough content to read legally? Yes, in the sense that I can import raw manga, but you can't expect everyone to do that.
After I've read scans of it.

The main issue isn't the translators.
It's their bosses, who tell them to change/cut/censor stuff.
(also, the translators are often worse than scanlations, because the bosses tend to hire the worst/cheapest translators)
Unskilled scanlations are bad.
Unskilled commercial translations is terrible. (if they're gonna charge for it, they need to ensure it's good! Also, if they're gonna pay the translator... same thing there)
...but intentionally changing/censoring/cutting, and making the work something different to what it is in its original language, but selling it, claiming it to be the same as the original (just in a different language), is a lie. It is fraud.

A lot of pro translators are not at all better than me
...and I'm nowhere near good enough, to be a translator.
If you think the translations are fine... That's because you are blindly trusting the licensing companies, and have no ability to verify their translations.