Mandatory heating rental in new buildings - is that possible?

I wouldn’t do it either. Apart from the high costs, for me having my own house especially includes a piece of independence, which I wouldn’t have here for a long time in an important matter.

own house for me, especially a piece of independence

Some would call that comfort! :D
An apartment rental also has its own comfort. You just have to be aware of it. Also independence, because locking the door behind you and coming back weeks later has to be organized with a property/ detached house.

But I agree with the OP mathematically.

However, I would do it if the numbers add up for me in the end. Because if other options (plots, house construction, BT offers) are more expensive, then I say frankly: go for it.
Others build granny flats for many times the subsidy and call that freedom, others again equip their house with €20,000 worth of comfort features to be independent (ventilation, automation), whether you can always balance that out in numbers, I doubt.

I wouldn’t do it either. Apart from the high costs, to me having my own house also means having a certain degree of independence, which I wouldn’t have here on an important issue for a long time.

Yes, I see it the same way. Personally, I would feel uncomfortable having built a house but still not being the "master" of my own home, especially when it comes to the building services. And to have to pay for this feeling of dependency for decades.

How are the 8 cents/kWh calculated? Electricity? Actual heat?

For the rest... well, a geothermal drilling would cost you a similar amount privately. The heat pump including underfloor heating also costs you around 20-25 thousand euros. Or do you pay for the installation of the underfloor heating yourself? It is a classic "all-round carefree" package. If the heat pump breaks down within 10-15 years, the municipal utilities will provide a new one and you are in the plus while the municipal utilities are in the minus.

It's like with a car. Leasing is a bit more expensive, but you drive carefree because all repairs are included in the lease rate. Whoever prefers to buy their car themselves and drive it for 10 years... they might be lucky... but even a medium-sized damage throws this calculation off. And the service provider charges for this security, we all have to make a living from something...

A geothermal drilling would cost you a similar amount privately

The OP wrote ground collector, not drilling. That should be significantly cheaper… although I didn’t understand whether it is laid on the own property or somewhere else. If it’s not a trench collector, I wouldn’t want it on my property anyway, then you can’t plant trees on the area.

The OP wrote ground collector, not drilling. That should be significantly cheaper… although I didn’t understand whether it is installed on their own property or somewhere else. If it’s not a trench collector, I wouldn’t want it on my property anyway, because then you cannot plant trees on the area.

The ground heat collector is not much different with drilling. Everything is significantly more expensive than 3 or 4 years ago. I think the problem here is that people rely on experience and especially prices from acquaintances who built before 2020.