"OK this is disheartening! And to think I was deciding between CO/Delta/AA and now nothing. I'm still shocked that CO wouldn't let me pay $1k to sit in a seat when they are all empty."

Probably because if they did that, those few people who paid the $5k for their tickets up front would have bought economy tickets and paid the upgrade at the gate. Having these people pay full price is the justification for a first class cabin in the first place. On a domestic flight, you might find people receptive to cash offers, but the international upgrade is a different beast.