If your income goes up 800€ and your taxes go up 800€ then your disposable income does not increase.

For those near the poverty line who will pay less than 800€ in new taxes still need to eat, so they cannot pay 800€ more in rent.

In most Western European countries rent is tightly controlled so it's not possible for landlords to make big increases overnight.

Ok: what keeps me from raising prices, period?

Also, rent was just one example. In the long run it seems inflation will just go up unless there's heavy regulation of the market, at least on basic goods and services (housing, food, clothing, etc).

Nothing keeps you from raising prices. However, just because people have more money does not mean competition will stop. In fact competition might increase and prices go down, as there is more money to be made and risks/costs go down.

Is it? As far as I've understood, rent control is quite rare in Western Europe nowadays, and at least Finland doesn't have any.

no it isn't, and where it is just causes havoc, ie you cannot beat market forces of many well-earning people wanting to live close to work with some law