Insulation in the 70s compared to today, modern insulation, heating costs

The other way around. The piston expands more; because the cylinder is water/oil-cooled (which is not so easy to do with the piston). Furthermore, it is basically doubly bad because of this: The cavity becomes smaller due to the expansion of the cylinder (the expansion happens uniformly in all directions), but the piston that moves within the cavity also becomes larger.

Ok, that makes sense. At least for gas.

This applies to all heat generators, not just gas. I remember when the first modulating gas condensing units came to the market. Especially two manufacturers tried with various publications to portray full modulation as unnecessary or dispensable. Today, these manufacturers also offer modulating gas condensing units

....With a modulating heat pump, for example, it should look different.

Which modulating heat pumps do you know?
If the annual full-load hours of a heat generator are << 2300..2500 h/a, one should consider a few things.

Best regards

Which modulating WPs do you know?

Mine does.