Push notifications have a cost too, now you need the server to maintain state of all clients and notify them on changes. This has huge implications at scale, e

Push notifications have a cost too, now you need the server to maintain state of all clients and notify them on changes. This has huge implications at scale, especially if it's extremely cheap or easy for new clients to spin up. Think about every single browser tab now requiring a server to maintain some state on the backend--that's a nightmare with billions and billions of users and tabs. Pull-based architectures have the nice effect that less popular content has almost no cost to continue serving and just falls into the background forever.