kudzu

I think that's true only in the front cabins. Try flying in SQ Economy (excluding Exec Econ which has more legroom and stuff); while marginally better all round, it ain't nothing to write home about. The majority of SQ pax connecting to US for LAS and other places on package tours would probably view US as a step down, but not three steps down.

SQ's Raffles and FC are rated highly year after year, for good reason - it's as good as it gets for folks willing to pay the hefty prices. And SQ prefers to let the front cabins fly empty rather than complimentary upgrading even their own Elites. The luxury of SQ up front only applies to those select few able to pay (or have the company or client pay), and to the rest of us who save our Miles for once-in-a-lifetime trips.

Yes, SQ has invested a lot in their front cabins, but they also have a cheaper labor force, etc which gives them a cost advantage over any US legacy airline. Back in its heyday, US tried a real First Class transAtlantic (the present Row 1 seats) but found it was economically unfeasible. Others may be willing to pay for premium travel, but not enough USA-originating pax seem to want to.

I would be surprised, though, if SQ execs had not actually flown US thoroughly to determine a "good fit" before agreeing to the codeshare. I wouldn't be surprised to hear that SQ had extracted a memo of understanding from US regarding codeshare-needed changes to the FC and main cabin - e.g. Mandarin/Cantonese-speaking FA on LAS flights, hot towels in FC, whatever.