Even a functionally equivalent system will have problems. For one, if you're absorbing a significant portion of the sun's radiation, that's going to generate a

Even a functionally equivalent system will have problems. For one, if you're absorbing a significant portion of the sun's radiation, that's going to generate a lot of heat that you've got to dissipate somehow. Then you've got to transport that energy in a way that's more efficient than the way the sun is already doing it. Then you've got the logistics problem: it's easy to build a modern CPU (for example) if you're assuming all the infrastructure already exists. If you're assuming it doesn't, it takes much more energy to produce the first one than any subsequent ones. And it may be the case that in order to build an energy-positive Dyson sphere/von Neumann probe network you'd need to already have consumed a couple stars prior. And to do anything on that scale, you're already talking about coordinating activities across multiple planets or solar systems, which comes with it's own massive overhead. There's a lot of ground to cover between burning fossil fuels and consuming stars, and little reason to assume a successful civilization must do the latter. You could potentially go quite a while syphoning fuel off a gas Giant, for example.