Uh.... I don't know what the issue is here?

I've SDC'ed at least a DOZEN times this year, often with 100% confirmed upgrades, and I've never had any issues being put on the waitlist, or even, if U space is available, being immediately confirmed in F.

I always call in, though. And it's always just worked (tm).

Maybe you should just do that. You can even ask them to check for U space for the flight you're about to SDC into, etc.