US carriers started many years ago with the idea that, if you upgraded your regular pax, they would like you more and be more likely to buy in C or F to be sure of a seat, as well as telling your friends.
It is my understanding that this got rather over toppled by the desire of folks for a bargain (who'd have guessed?) and that the expectation of the upgrades became paramount. Many DID promise this but found the burden of upgrades and FFMs could grow far more than they expected.
European carriers started down this route (pun intended) and then pulled back. But I sit to be corrected.