Here's my argument for why leader swapping is superior to civ swapping (in no particular order):
1. Leader swapping has more support from previous Civilization games
Previous Civilization games have had multiple leaders for the same Civ. This at least provides some precedent for allowing the player to switch between different leaders. Civ swapping has precisely one element of support in previous games (Eleanor of Aquitaine).
2. Leader swapping is more realistic
The premise the developers provided in their (mostly abysmal) reveal last week, was that Civilizations transform into something else when faced with a crisis. This is only really true if you look at the crisis as one of being conquered, subjugated, or colonized. Anglo-Saxons became English because they were conquered, not because they were faced with some nebulous crisis.
It is far more in line with human nature to look to a new leader in a time of crisis, rather than wholesale changes to ones culture, traditions, language, etc.
3. Leader swapping provides for more interesting gameplay
I fundamentally disagree with the developers' reasoning for adding civ swapping into the game, which is to always have civs that are well-suited to the time they are in. I believe this is a huge misunderstanding of their own game. Since Civilization 3, when we first saw differences between civs, a core challenge to Civilization gameplay has been navigating times where your civ lacks advantages and capitalizing on times where it has advantages. It makes for interesting gameplay when you have to take drastic actions to bring the situation more in your civ's favor.
With leader swapping, you would have choices of different leaders, but no guarantee that any of them are perfectly suited to the time and place your Civ is in, keeping a core challenge of Civilization in the game. Civ swapping, as proposed, removes that challenge.
4. Leader swapping removes what I believe is problematic cultural commentary
Humans are not interchangeable widgets which easily transition from one culture to another. The idea of Egypt becoming Mongolia or Songhai without being invaded and subjugated is proposterous.
5. Leader swapping allows for roleplaying or creating a new history as a civ, which many players enjoy
This isn't particularly important to me, but many players like having modern Romans, Babylonians, Carthaginians, etc. Civ swapping removes that aspect of gameplay.