Affinities would change the gameplay dramatically. Every hero, every item and every strategy would be totally different from what they used to be.
The changes in MOBAs are created gradually. For instance, a new item, a new hero, the remake of a spell etc. This is to make the game coherent. Remember, Dota 2 is an e-sport with millions of dollars in terms of prizes every year. A dramatic change like that would turn everything upside down. When your product produces money you don't change the recipe. It's like suddenly changing the very core rules of Basketball. You can't. Not when you have millions of players and hundreds of millions in revenue.
A dramatic change like that sounds like Dota 3 to me or even a whole different game.
To answer your other questions:
Don't you think it'll elevate the gameplay in terms of drafting? Other than just timings, don't you think affinities will provide more depth in the way spell combinations are being executed?
This is a personal opinion but YES, something like that sounds awesome. Nevertheless I wouldn't expect a dramatic change like that for the reasons stated above.