2^53 is 9*1000*1000*1000*1000*1000 it's over 9000 trillion messages. What is the name for 1000 trillion? 9 million billion. Maybe this is like the moment when some IBM guy said "I think there is a world market for maybe 5 computers" or bill gates said he couldn't imagine why anyone would need more than 640k of ram. But I think this will be fine.

To get to that number in a year, you'd have to create 3 million messages per millisecond. Or, one message every 100 seconds for the age of the universe. If that happens and someone is still following you, you have to start a new feed. sorry!

(in fact, I wouldn't be against a setting a lower message number limit, such as uint32)