The obvious middle ground was having Civs stay the same and each Civ has a pool of 3-5 leaders which can be switched to when transitioning to a new age. Instead, they chose the most extreme implementation and here we are.
As leaders are basically bonuses, like an extra ability for a civilization, this could make perfect sense. You don't even need five leaders, make it three per civ - one leader who was a "founder", another leader from a time of growth and expansion, and a third leader from a more "modern" era. The early leader, regardless of when he was alive in real history, will always have bonuses of a "founding father" - exploration, early expansion, founding a religion, etc. The "middle" leader will have bonuses to strengthening you civ, while the "modern" leader will have more late game bonuses.
It can work for pretty much every civ.
The Americans can have Washington as their early leader, Lincoln as their middle leader, and Roosevelt (Teddy or FDR) as their modern leader.
The English can have Aethelstan, Elizabeth, and someone like Queen Anne or maybe even Churchill.
The Romans can have Julius Caesar, Trajan, and Constantine (or if you want, make Constantine a middle leader, and some Italian leader like Lorenzo or even Garibaldi as a modern one).
So you take a civ's history and just divide it into three parts, and take a leader from each part.