the manufacturers have only themselves to thank for this perception that seat-mile costs are the be-all and end-all of life
I've never come across an airline that laboured under that misapprehension. If they did, they would all be flying A380s.
There are some circumstances when SMC is the most relevant metric, and other circumstances where aircraft-mile cost matters more.
That said, almost all airlines calculate their yield on a seat-mile/passenger mile basis.