When I last flew CUN-PHX all the upgrades cleared and we left with an empty seat in F. But this was before upgrades were complimentary so it's possible that there were elites in the back who didn't want to buy/use stickers.
If you are flexible with travel dates and book the cheapest flights you can find you will get upgraded a lot more often (as a nice bonus to saving money on your tickets). Those cheap flights are usually cheap because they have low loads.