"how much it cost just to gain those ratings that you're likely never to use after your first 121 job"

I don't know, man. Some of us love aviation. I've been doing 121 since 1988 and have maintained my CFI. During that time I've signed off 50 or so IFR ratins, ran a flying club, and currently own three GA airplanes. For those that love GA, the CFI can really be nice to have.