I'll offer just one narrow comment. Delta has space-available unlimited domestic upgrades, but not international. No airline gives international business or first-class upgrades simply because you have high status. I don't know Delta's rules but I think they, and most airlines, offer a limited number of systemwide upgrades per year.

Even as a diamond Medallion you probably wouldn't hit 90% domestic upgrades. Most carriers are selling more discount F fares, preferring to get some money for them rather than giving them to elites for free.