Is heating maintenance really worthwhile?

Yes, maintenance should be carried out regularly every year. Otherwise, there is no guarantee. Like with a car.

That is not entirely correct. Those who have signed a contract according to VOB for the installation can obtain a guarantee through such a maintenance contract that extends beyond the 2 years warranty according to VOB. Those who, on the other hand, have commissioned the installation according to the Building Code usually also receive the 5 years warranty according to the Building Code for "buildings" (depending on the scope and type of work on the heating system; in this case very likely given as a complete new installation).

Whoever, on the other hand, has commissioned the installation according to the Building Code usually also receives the 5-year warranty according to the Building Code for "structures" (depending on the scope and type of work on the heating system; here very likely given as a complete new installation).

Anyway... we have it according to VOB, they think the same way... the rest is also "according to the Building Code" in the fine print ;) We don’t have a maintenance contract. Quote plumber: "remember, always in winter, we’re happy to come." That’s how I do it with cars and so on. No commitment, free choice :) But that doesn’t matter here: a maintenance doesn’t take long. And if your wife stood next to the whole time, I do hope she offered a cup of coffee.

we have according to VOB

You probably don’t either; it requires appropriate clarification on the part of the contractor. Simply writing "offer according to VOB" is not enough. But if a private client has actually agreed on the VOB, one should consider whether the "free choice" and thus a few hundred euros saved on maintenance costs are worth losing 3 years of warranty.

By the way, this also applies to cars – if the independent workshop doesn’t work exactly according to the manufacturer’s specifications, the warranty is void. Any goodwill gesture from the manufacturer after the warranty expires can be forgotten in any case.

And no, a proper maintenance doesn’t just take 30 minutes (possibly including coffee). This belief might be widespread because it is often handled that way, but it’s just customers being fooled. Even more so if you voluntarily throw the manufacturer’s warranty out the window for such work.

Of course, everyone can do as they please; personally, in both cases (heating and car), there are simply too much money at stake for me to take a risk over maybe 500 euros.

What kind of person are you?

You probably don't have that either; it requires appropriate clarification from the contractor. Simply writing "offer according to VOB" is not enough.

I know what we agreed on. Certainly not because of a heating warranty,..

I don't know how many maintenance services you have experienced... I have already seen more than half of my age... and we don't want to claim that none of the guys did anything ;)

What kind of person are you?

Someone who has contradicted you. It would be nice if we could keep this on a factual level.

I know what we agreed upon

I did not mean to imply that you didn’t know what you agreed upon; I pointed out that there is a huge difference between “I signed it like this” and the legal evaluation of this action.

And sorry, but the quantity of maintenance work you personally experienced really says nothing at all about its quality.
I could also claim to be able to assess the quality of a photograph because I know a dozen people with a DSLR... and they surely can’t all be nonsense ;-)

Someone who contradicted you. It would be nice if we could keep this on a factual level.

Factual level is good. You did not contradict me, you accused me of not knowing the basis of our contract. That is a difference, and that is what my inquiry refers to.
I'm out of here :D