So.. what... heatsinks are expensive, so are mosfets. manufacturers use the best combination of the two to get the lowest cost. nothing new.
On the other hand, are you trying to reduce cost or improve efficiency? or both?
In addition to these reduced I^2R losses you should look into utilizing silicon carbide diodes, and redesign the subbers for the reduced recovery times and lower capacitance. Of course you may not need those, depending on what topology you are using.
I think you are ignoring switching losses, unless you wish to square wave drive an iron transformer at 60Hz, but who would want to do that?
Please provide more information.