The fact that we don’t see the light of stars, or whole galaxies replaced by dim infrared waste heat emissions means either (1) new physics will point to free energy solutions that circumvent thermodynamic laws [unlikely], or (2) we are the first.
Which seems to be the most likely explanation to me.
1. disassembling the stars to make them live a thousand times longer
2. it only takes one expansionist civilisation seeking interstellar exponential growth — even if 99.9% of civilisations are quiet, that's just 3 digits of the 22-digit number of stars in the observable universe.
Why would they do that when there are insane amounts of start with billions of years of life ahead of them. What purpose would that serve at this point in time?
In that case, the stars are not 'waste'.
Historically, this idea of growth as method for advance is only a few centuries old. It's not hard to see its end as efficiency keeps increasing.
Going K3 to 100 billion stars of the galaxy "only" gives you a 10^11 constant factor speedup at enormous expense and with Amdahl's penalty at galactic scales.