I think these flights are often oversold in Y, as most of my flights have included last minute op-ups from C-F for 1-2 lucky folks (presumably people who actually paid for C) as well as 1-2 Y passengers who didn't pay for upgrades, but got to ride in C (they always seem more excited than the C-F passengers - wonder why
).
So you can get into F by paying for C and hoping for an op-up, but they won't do it to fill F, they'll only do it to make room further back.