Lemmy.world isn't meant to replace Reddit. The fediverse and various implementations of the ActivityPub protocol are. Lemmy and Kbin appear to be some of the frontrunners, but lemmy.world isn't a lynchpin. Other Lemmy instances are up. And it's a bit naive to expect mostly volunteer servers to have the same uptime as a well-founded corporation. And let's remember, reddit used to go down a lot more early in its life compared to late in life (and it'd still go down then).
Plus, it's still a bit entitled to expect uptime just because they have an intent to provide what they believe is a better service than reddit. It's not only about folks ingesting content, but also those who create and moderate.
It's reliability has been pretty good all things considered.