Not sure if you're talking about SAC or SJC... SJC-MSP (198/200) and MSP-SJC (195/197) are usually quite full. They used to have SJC-DTW, but they discontinued it a while ago (2001?). Anyhow, I think it's long past time to bring it back. CO's SJC-EWR is almost always full, as is DL's SJC-ATL. I absolutely hate having to do SJC-MSP-DTW-whatever (which I do sometimes have to do), as the MSP-DTW is always a crapshoot for me. There's like 3 flights within five minutes, inevitably a number of pax are waiting at the wrong gate, and I've seen the flight delayed a few times more than I care to recall.

As for SMF-MSP (394/398), been a while since I've taken either of those so I cannot comment on load.

Personally, I really like the way CO handles SJC. Lots (relatively) of flights to IAH, which let you make reasonable connections, and then a late night flight (redeye, dep 9:30p) to EWR. DL does similar, lots of flights to SLC and two flights to ATL. (Unfortunately, no flights to JFK as far as I know.) CO and DL certainly seem able to do OK with this model, and so personally I would love to see SJC-DTW return. I would love to see NW ramp up at SJC - perhaps even 2-3x MSP, 1x DTW, 1x MEM?

edit: wrote sac instead of smf. oops.