For example, also because of costs.
You recoup that relatively quickly on a calculative basis if you are aware of the follow-up costs over the usage period. Not to mention the living space wasted for setting up the radiators.
Your talk, that you don’t have to build at all without underfloor heating, no one can believe that anyway.
You don’t have to believe it, as it can be explained factually.
A gas heating system also saves energy if it is only required to deliver low flow temperatures, and the argument about the cheap fireplace only applies in the end to people with their own forest (although there are opportunity costs even here).
The "flexibility" might be accepted, although I can’t think of any sensible scenario in which one would be dependent on it.
For warm extremities, one can always set up an electrically operated radiator if necessary, without condemning the whole house to timely renovation. But that probably belongs more in the thread about building mistakes.