You're absolutely right, that is the way to get lowest standby power, which is potentially very important. However, as you know, the jack_the_lad cheapskate way is to just shove a 15W DCDC battery charger on the output of the offline PSU.
And many an engineer is often kept out of the purchasing loop, and so wont know the cost difference between doing this with a custom design, and doing it with off the shelf power supplies. And ditto that, but this time take the situation of a company that already owns its own SMD soldering business, and already does other PCBs (non-power) with that, (company populates its own PCBs with its own SMD machines).....how does the custom_design vs offtheshelf conundrum work in that case.
Its always been a mystery to me what the actual price differential is between custom design and offtheshelf buying. But thanks Easy Peasy for your post#2 above where you give actual figures.
I mean, i wouldnt mind betting that eg Meanwell , if one showed them significant volumes, would offer a phenominally cheap offtheshelf solution, and even modify one of their existing offline, open frame 150W solutions to suit.......and i almost bet my shirt that they'd viciously price it well under the cost of any company doing it as a custom design.......but as discussed, no engineer would know...as final purchasing "deals" are always done at the top level in a company, and the engineer hasnt got a clue.
I am always amazed about the way managers in companies always ask what is the price of a custom design for xxx Watts and xxx volume....when no engineer would ever know this. (unless they owned their own consultancy)