He's too busy making motorcycles to film more (he loves bikes). I think he's at a happy place. If he just did one quality John Wick movie every 18 months I'd be in a happy place too. Think there's enough story there? I bet there could be.
Lots of pets to go through, cats, fishes, birds, iguanas, gerbil, rabbit, etc...that gets killed each movie and he goes on a revenge killing spree, and gets a new different pet at the end of each setting up the next one.
Also gotta remember the first one totally flew under a lot of people's radar. The real hype started with 2, which is probably why they repeated a lot of the story beats from the first one. And it was a full-up franchise by #3 so by then people knew what they were getting into.
I'm still hyped for 4&5 but I'm guessing after that, enough of the mystique will be gone that rest of the universe will not be nearly as compelling. I'd love to be wrong though, if they can continue to produce compelling action movies with a minimum of bullshit.