Ventilation in new buildings (central vs. decentralized), noise disturbance

Why should I ventilate when I have a ventilation system specifically for that?

That's true. Best not at all. However, the OP "likes to keep the windows open" and therefore in my opinion does not need an expensive ventilation solution.

P.S. One more question about the extensive maintenance. What do you mean by that?

Well – every system needs to be kept clean and maintained. (Filter replacement, cleaning/disinfection...) The bigger, more expensive and more complexly built it is, the higher the follow-up costs. For some (hay fever sufferers) it's worth it, for others (ventilating country dwellers) it just isn't. My decision was also strongly influenced by the additional cost of about 10,000 bucks (which would only have paid off after almost 30 years)... It's important not only to compare the pure system costs but also the changes in the statics, ceiling height, cutouts in coverings, etc.

-more complex planning including suspension and boxing of the pipes
-(I heard from a supplier of decentralized ventilation systems: As soon as I turn up the ventilation in the living room, more warm air is blown into the bedroom --> not desirable)

For example, when laying pipes in or on the raw concrete ceilings, nothing has to be boxed or suspended. And you can't just "turn up" one room there – everything is always "turned up".

Why would you insulate a house thoroughly and seal it airtight, only to then drill holes in the exterior walls? I will never understand that with decentralized devices in new buildings – not to mention the noise factor.

Why should you insulate a house extensively and seal it airtight, only to then drill holes in the exterior walls?

Because otherwise you suffocate?! You need fresh air with all systems, after all. I see little difference there now.

That referred to decentralized systems (and especially) without heat recovery...

Cleaning can usually be done in a few minutes per month. I suspect that with open windows, more dirt enters the apartment, and therefore the cleaning effort for the house itself is higher. Probably very few people will leave their windows open permanently, i.e. when you come home after a few hours: really stale air (I once turned off the ventilation system for a few hours as a test just for fun --> I will never do that again ). At first, we wanted to choose the solution "central exhaust + decentralized supply air," but for various reasons, we decided on "completely central." Despite the lack of a basis for comparison, I am very satisfied with this option.

That referred to decentralized systems (and especially) without heat recovery...

My answer as well. Why should it be better to have one big hole instead of five small ones?

Regarding heat recovery, I have already said that with heating costs of 1000 to 1500 per year, it is hardly possible to save enough to make heat recovery cost-effective.