I see your point but I think you misunderstand me a bit.
r/all is somewhat of a "front", it can show that the place is vibrant and that there is something to do and something to scroll through.
But for example I never saw anything from r/mealprepsunday on my r/all and when I had reddit it actually was an subreddit that I was subscribed to and used frequently.
If the subreddit above gives me a reason to stay on reddit and it would dissapear then an vibrant r/all isn't an incentive for me to stay.
Can you see where I am coming from?
a page like r/all has it's use. It can reel people in and give direction but it will not be the reason people will stay.