News flash: X is already an echo chamber!
Maybe the "oh no don't leave for an echo chamber!" assertion is a case of people remembering the pre-X version of twitter, and comparing competitors to that - rather than what X is today. Or maybe it's a case of "boiling a frog," where people on X are actually unaware of what it has become.
Or maybe... it's actually just a bad faith argument by people who like the current X echo chamber.