As stated by others you should not expect to get upgraded just as a "nice gesture" because business/first seats are available. It's a different story for oversold flights though. In my experience they will always fill up the plane in all classes and do upgrades as necessary (not only for confirmed passengers but even standby). So the upgrade chances really depend a lot on the route, season and other factors that affect the load.
Having been with TG for >25 years i got upgraded countless times, especially in the good old times when they were flying the A340 to Europe. That plane had only 199 Y seats but 60 C and 8 F. That meant that for Saturday / school holiday flights where majority of travelers were Y they always had lots of upgrades. I remember a time where i did a Europe-BKK or BKK-Europe every week - always on Saturday - and during the 3 months i did that i scored 9 upgrades out of the 12 flights !
These days things are very different and i rarely get upgraded. In my opinion the main reason is seat config: a 777ER has 42 C and 306 Y, i.e. 14% premium seats - compared with 34% on the A340. They maybe also have better optimized inventory management and other factors that reduce op-up opportunities.