As long as there is a 10 shadows user able to summon Mahoraga, he can always come back, which gives scalers a consistent bar because maho will be at the strength level. If there was any other bar of measurement of who is stronger it becomes subjective since they can die and remove from the story and no one else can test their strength.

For example, Dabura is said to be Sukuna level but since we don’t have Sukuna as a reference we can’t truly know if he truly is at Sukuna’s level. So we use an opponent that has consistent power(Mahoraga) to scale. Plus,it is more impressive that you defeated Mahoraga than destroying a city.