I wouldn't say it is quite 0%, but close to that. Operational upgrades do sometimes occur. However, those are usually given because the flight is overbooked in economy class but there is space available in business class -- i.e., to meet the airline's needs. Also, on United, the op ups are usually doled out based on factors such as elite status. If you're a no-status Mileage Plus member, your chances are probably very small.

Most people who get upgraded use miles or certificates to do so, but they are still on a "space available" basis -- no guarantees. Here again, your status is a factor on the upgrade waitlist.