But you need 40% of your "users" to qualify. If I take all registerd people on my platform as "users", I might end up at 0,1%. But if I define a user as someon

But you need 40% of your "users" to qualify. If I take all registerd people on my platform as "users", I might end up at 0,1%. But if I define a user as someone who is currently using my product >1 month, I might end up at 60%. You are correct that not all products can have retention. But if you do, the cohort is a better metric in my opinion.