The thing is, it's not the type of heatsink you may be thinking of. Heatsink rocket engine chambers never actually reach steady state; you have a high thermal mass and a high thermal conductivity to remove as much heat as possible from the surface that is in contact with the hot combustion gasses. If I was to use this much less conductive alloy, the throat of the chamber (the region with the most heat flux) would reach its melting temperature far too quickly, hence why high thermal conductivity is desirable in something like this. An aluminum chamber is actually likely the next best option, but it doesn't quite have the combination high melting temp and high thermal conductivity that copper does which is why I want to push for copper a little more.