Well it is a theory. The problem with upgrades is that it's difficult to really start to have a somewhat accurate idea as to the chances till quite close to the flight so even for high status travellers there is a risk of missing out.
Some would try to book off-peak times of the year, outside school holidays and on a less popular day of the week to travel for Business.
It's about picking the right times.
Getting an upgrade could potentially be more difficult at peak times if there are more high status travellers who usually travel for Business and are used to Business Class travelling for leisure and looking for upgrades.