Main water pipe through our garden - overbuilding?

Just simply request in writing to have the cable routed. Let's see what happens.

The community or the water supplier has to expect that trees might be planted in a garden.

What was there before? The pipe or an established garden? I don't mean any harm to you here, but: You should think neutrally and sensibly again and not just stubbornly and obstinately in one direction. Go and clarify this at the table together with the office. Bring the plans with you and show them why they are being disturbed.

Or is it negligent to operate a water pipe on private property without securing it? After all, it's my land. The municipality or the water supplier has to expect that trees might be planted in a garden.

Then plant ten oaks exactly where the pipe runs, and put your garage on top of it. Just wait and see what happens.... You apparently don't want to hear anything other than your opinion being confirmed...

Go there and clear it up at the table together with the office. Take the plans with you and show them why they are disturbing.

This will be difficult. I have already been to the town hall several times on the topic and have submitted proposals. More than the already mentioned offer never came out of it. I had even offered them an easement (and that would have taken the tree issue off the table anyway), but there was also resistance there. The property was once an orchard meadow (before the line), then a fenced lawn, and now just building land.

You are not the first with such a case and to my knowledge in a dispute it always ends with the easement being forcibly registered in the end and that’s it. The justification more or less always comes down to the prehistoric viewpoint ...Gemeinwohl geht über dein Wohl... hinaus.

We only seem to be hearing half the story here, that's the feeling I get. What is it exactly that you want from the community and where are they being so stubborn? Does it actually help you to switch to confrontation? The art is more about getting people to want what you want.

What kind of trees are supposed to be planted? Fruit trees I see as completely relaxed. You hardly want to chop down a 20-year-old lime tree when it really matters.
Although:

... before the tree destroys everything you could just plant it two meters further over...

is of course complete nonsense. What is above the ground is also below the ground. So if the crown has or will have a circumference of 20 meters, so do the roots (extreme taproot trees excepted).