Why is Modern Affinity called "Affinity"?
The answer is #3.  i.e. There was a deck long ago, that exploited affinity.  That deck has slowly changed over the years as the card pool has changed.  As those changes have occurred, the original reliance on affinity as a mechanic has reduced, but the name has stuck.

References:

https://www.mtgsalvation.com/forums/the-game/modern/established-modern/aggro-tempo/219590-affinity

https://www.channelfireball.com/articles/deck-of-the-day-true-affinity-modern/

https://blog.cardkingdom.com/deck-week-modern-affinity/