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Up until a process generation or three ago, costs were declining as efficiency and performance increased!

High absolute performance (Xeon / Epyc scale), maybe.

High performance per watt, why not all three? Smaller transistors take less power to switch, so they likely can be run at a higher frequency without thermal issues. (Of course, a lot of other design considerations apply.)

When I compare my phone with a PDP11 it seems obvious that I can pick all three and then some

You are comparing time series data when the statement was obviously meant about cross sectional data.