Geelong's sustained run in the top part of the ladder has been remarkable, no doubt.
However there have been far greater achievements and examples of sustained success over the history of the VFL / AFL using other, more significant measures of success.
For example both Collingwood (1927 - 36) and Melbourne (1955-64) won six premierships within ten year windows.
Richmond won 4 from 8 between 1967 and 1974, and as mentioned Hawthorn 4 flags between 2008 and 2015, Hawthorn also having won 5 flags in 9 years from 1983 to 1991.
The argument against those old achievements is always that there were fewer teams, no salary cap etc. Which I accept, but also note that at least in theory, other clubs in those eras had the same opportunity to build amazing records. For example, what was to stop the other 11 clubs getting to the level of Melbourne in the 50's? If it was simply that Melbourne was by far the best run Club, best coached and looked after its players the best, then the inability of their competitors to match them does not diminish their achievement.
I reiterate that Geelong have had a fantastic era, and have stayed close to the top despite almost completely turning over the 2007 flag team. However, to me unprecedented means best ever, and their era is not that.