Since the game is not getting any more balance updates, shouldn't all previsous runs (unless hero didn't recieve any balance change since the record) be considered obsolete?

It's a little bit more complicated. For example Towers of Doom. On version 2.53.2 the fort has 12 900 hp. In the current version 2.55.3.89754 the fort has 15 450 hp. If you choose Sgt. Hammer you will make an additional 14-11 attacks to capture a fort. Depends on the hero's level. 1 attack equals 1 second. 3(forts you need to conquer)*12(attacks)=36 extra seconds 4:52 on version 2.53.2 ≈ 5:28 on version 2.55.3.89754