Being open source won’t prevent this, sadly. 4 years is still young, but if a critical mass shifts back to Reddit then Lemmy will be considered a failure.

you express very limited understanding of open source and how competition works. Just because Microsoft kept selling Windows and “Linux on the Desktop” never came to displace Windows, it doesn’t mean Linux was a failure. Android came along and is the biggest Linux distro ever, defeating Windows CE / Windows Mobile.