Re: Past and present
Hold on, this is not an assertion but one of two different theories that have been offered to explain the observed data from New Horizons.
Your observations about the conditions on Pluto may indeed be correct, but there is evidence of the movement of the Sputnik Planitia within recent geological timescales. That and the visual absence of cratering on the surface of the Planitia, which is accepted as evidence of the activity of some sort of process that has recently (geologically speaking again) reshaped the area, leaving a smooth surface.
Some sort of activity is happening out there, that is apparent, and that implies a source of energy to drive it. This is just an attempt to explain that, not proof.