I've never heard of home maintenance budgets being more than, say, 10% of the cost of a mortgage, and usually you're advised to budget only a few percent. Obviously there are outliers like when you buy a run-down old ruin and have to do a complete period-correct refurbishment, but hopefully people know what they're getting into in those rare cases.

Just replacing the heating (very common, eg lots of oil heating in Western parts) costs near 10%.

Doubtful. On our £250K house stripping out the old heating system and fitting a combi boiler cost £4K, which is about 2%. Rewiring the house was another £2K. A reminder that you claimed above "Except owning a house costs way more than the mortgage itself", so even if all this stuff costs 10%, it still vastly cheaper than the mortgage (even of just the mortgage interest).

I have been a home owner for roughly 20 years by now and I don’t think I spent more than 5k euros in maintenance so far…