I don't think we're actually disagreeing as much as you think we are, and perhaps that's my fault for making nick-picky points that muddy the discussion boundaries. Let me try again.
There is some physics-based limit to the amount of energy that a civilization can control. This is different from the practical amount they WILL control, which is driven by any number of other factors. There are several cases:
1a) That limit is >K1 and our knowledge of possible intelligences and physics is sufficient to know something about advanced civilizations. The limit of energy acquisition is NOT the Great Filter, since we should observe the results. The fact that we don't see Dyson Spheres everywhere means that some other factor is the Great Filter.
1b) That limit is >K1 and we don't know or misunderstand something important about possible intelligences and physics. The whole Fermi Paradox is badly conceived because we are overconfident monkeys making too many assumptions.
2) If the limit is <=K1, then physical limit of energy acquisition is the Great Filter. We'd better make the best of what we have now.
Most likely we are dealing with case 1x), since if you hand-wave away a few millennia of engineering there's no reason we know of that those things should be impossible. I don't think it's a given that we are in 1a) and not 1b).
Stepping outside of Fermi's Paradox a bit, even a hypothetical K3+ civilization will eventually be forced to "surrender to entropy". That is the outcome of a survival-maximizing game. I don't think it's very useful to define "surrender to entropy" as "failure", since that's the BEST-CASE scenario. You're free to find that a trivial or distracting redefinition.
