I can certainly understand the frustration at being told the lounge is too full, but in the case of the SQ SFO lounge, it is very small.

When I was admitted a few weeks ago, I came in around 11am (IIRC), and it was pretty quiet. But by 1pm it was getting very full, and by 1:45pm, there were no open seats (or perhaps one or two) in the business side.

I'd be interested as to what would happen if you were on one of the SQ flights and the lounge was at capacity - would they deny you entry, admit you knowing that there was no space, or try to kick out UA lounge lice? One thing I'd be pretty confident about - they wouldn't admit an SQ C pax to the F lounge if the F lounge was empty and C was full.