Pressure is force over area, and it takes energy to apply force. I'm simply pointing out that the pressure that instigates fusion in the sun doesn't come from nowhere. It takes some kind of energy to apply that pressure, that energy being the kinetic energy of the particles making up the sun, that kinetic energy coming largely from the particles falling in toward each other under the influence of gravity, and they didn't magically get more energetic while falling, they merely converted potential energy into kinetic energy.
You're right that at no point is any energy "consumed" in a sense that it is destroyed or any such thing. It is merely converted. Potential energy to kinetic energy (which in aggregate can be treated as thermal energy), which then does work in compressing the gasses of the sun, in turn igniting fusion which (among other things) converts some energy from rest mass to radiant energy, much of which in turn is converted back into kinetic energy of the atoms which absorb those photons, some of which in turn is converted back into potential energy as the gas expands, and so on in a cycle slowly converting much of both the potential energy and rest mass of the loose gasses the sun formed from into radiant energy until there's eventually nothing but a small, cold, dense stellar remnant, and a bunch of photons scattered about the universe, which all sums up to exactly the same energy as the rest mass, thermal energy, and potential energy of the gas we started with. Ignore the potential energy in that equation and it looks like we magically got energy from nowhere.
Even without fusion, the potential energy of the loose gasses would be converted into radiant energy as they collapsed, heating in the process, and radiated away that heat. What fusion adds is that we get a lot more radiant energy out of the sun than just the potential energy that was present in the loose gasses: we also get energy that was bound up in rest mass. Likewise, artificial fusion becomes useful to us when we can press atoms together efficiently enough that the rest mass that gets converted to radiant energy is greater than the energy wasted in the process of doing that -- energy which is not destroyed, but merely lost to heat, exactly like a collapsing gas that doesn't ignite fusion.
We can pump a bunch of energy into system and get a hot system that can then do work to generate more energy, but that's a waste unless in the process we somehow liberate energies that were previously stored in the system greater than the inefficiencies of that process. (Like what happens when we liberate the chemical energy stored in fossil fuels by heating them to the point of combustion). So we are looking for more efficient processes of liberating energy from rest mass; right now the energy wasted is greater than that liberated with any known method. A new method may tip those scales. But in any case, in order to liberate energy from the rest mass in a system, we will have to do something to that system, and doing something, no matter what that something is, requires energy be input into the system from somewhere. Not even the sun just magically spontaneously starts fusing, which was my original point.
