The old model was to charge for upgrades via certs and give you 4 500 mile certs for every 10,000 miles you flew. I recall this on UA in the mid-90s. In the mid-90s a few airlines which were in distress started programs of doing upgrades if you were on a top fare grade; i.e. a "Y-up" or "B-Up" fare (I think TWA was the first, and that had a lot to do with the weirdo deal left over from Carl Ikon's take over of them).

CO, shortly followed by NW, after this started offering " unlimited elite upgrades" to domestic first. The difference was the "unlimited" part. In CO's case it was because of financial distress, and NW then followed them. HP was close to CO/NW at the time, and I think they adopted their policies in an effort to get traffic from their elites. I am trying to recall when Alaska started giving upgrades, and as I recall it was in this very early time frame, as I recall getting an upgrade to Mexico in the late 90s. I think Delta was the last airline to offer them, and then it was a response when they went into BKR in the 2001-2 period.

UA and AA kept using certs, until the bottom fell out of the travel market c2008. However,by the mid-2000s both had started doing "Y-up" fares. But it was only the 2008 crash that finally caused them to give up on certs and go with "elite upgrades" They were losing too much high value Y traffic to airlines who would then put you into First.