A larger chainring/ cog combination is more efficient, I don‘t think anyone doubts this. For a super flat TT, a super flat road race or anything on the track, its best to choose a 100T Chainring with a 28T cog…
However, the difference between 53/39 and 50/37, both with the same chainline is around 1-2 Watts at 300W+.
My argument is, that the smaller chainring, paired with the SRAM AXS cassettes offers a very wide range, with very small gears at the low end. This to me is often more important than saving a Watt at threshold.
I have the feeling that the vast majority of road cyclists runs out of gears more frequently, than having problems from losing a watt or two. I am always surprised by how average joes tell me they have now upgraded to 11-28 for their trip to the alps…
From this brutal over gearing the efficiency loss is a lot more than two watts…