person A bought 1 000 bitcoins at exchange.
Price doubled so he wants to sell it but he is not going to just get money back to his account because he would have to pay high income tax.
He is going to sell it face to face.
Now someone bought that bitcoins but he probably wont sell it at exchange too because he also would have to pay tax, especially as the price again increased.
Which brings us to the conclusion that there might be less and less bitcoins for sale at exchange which will increase the price all the time forever?
maybe i forgot about something? or maybe it wont be that "bad"
And you certainly shouldn't hope for that, because what you're talking about is obviously unsustainable, and a deflationary doom spiral for sure. Deflationary spirals are already a possible problem in Bitcoin's far future, there's no need to ensure any.