Earlier quoted context omitted.
Why do you think they can't coexist? In fact my actual suggestion is that they should coexist. But the "just subscribe appropriately" response is fundamentally broken, and only provides a partial solution. This approach absolutely benefits any subreddit as well.
What exactly is fundamentally broken in subscribing to subreddits? If you are interested in fitness, you subscribe to /r/fitness. Fitness materials won't all of a sudden be posted to /r/gaming where you (if you are not subscribed) will miss them. So, what's the point of automatic recommendations on Reddit? If there is any fitness-related material on Reddit, I'll see it in /r/fitness anyway. Besides, how are you going…
"I like action movies" "Here's Megashark vs 747. You like action movies, so you like this"
How is any given subreddit any different? Being into fitness does not mean you share an identical interest/taste/discernment with everyone else who is into fitness. Further social news sites are subject to the tyranny of the minority -- aka the lowest common denominator. When people are more likely to express approval than disapproval, a small percentage of users with a common interest will tend to dominate.