As others on FT have said, if you really want first/business, then pay for it. Don't rely on getting an upgrade, it is not a certainty, especially in the current environment of shrinking upgrade inventories and an increasingly opaque set of rules governing them.
I wouldn't want to be in a situation where I paid a premium for a Y/B/M fare and didn't get an upgrade. Not only would I be stuck in coach when I really didn't want to be, but I'd also probably be sitting next to a V/X/T fare who paid thousands less for the same exact seat.