Part of the problem may be the bugs on the Lemmy instances which have not been showing "hot" content correctly. This may be giving a false impression of an obsession over reddit as posts are stuck on the front page?
I've been browsing a few different communities and am finding things are active, they're just not reaching the default Lemmy front pages. Sorting by new or diving deeper into communities shows a lot of new content.
For example I'm currently browsing from Feddit.uk and this seems to have exploded into an active community in just a week; it's really heartening to seen. I've also seen different content when viewing from Kbin based instances like Kbin.Social or Fedia.io (including from Lemmy.world).
These concerns are valid.
Some are transitory however - 1, 3 and 4 all reflect the current state of Lemmy and the similar Kbin are in currently. The Reddit issues were unexpected and people have migrated en masse to Lemmy/Kbin and have found was is in many ways Alpha software. This issues will mostly be resolved with time, and that is probably accelerated now as more people means more people interested in development, and motivated by anger at Reddit. I don't think Lemmy/Kbin will replace Reddit right now, but I think a new trajectory has been set. Communities are hitting critical mass to keep growing.
Look at Mastodon, it's at 1.2m-2m active users each month; it is still small fry and niche compared to Twitter but it exploded thanks to Twitter's mess, and is growing. I think we're seeing something similar with Lemmy and Kbin, but this is just the start of a long road and an expanded community will accelerate improvement and growth.
But point 2 is fundamental to the fediverse - fragmentation due to defederating could be a concern. I get Beehaw's motivation but I think their actions will consign them to a niche part of the Fediverse, but that may be what they want. Ultimately I suspect the biggest servers will dominate a main interconnected fediverse through sheer size and notoriety - new servers will need to federate to the big players to grow. It's not necessairly a bad thing - but people may end up signed up to a "main" large interconnected "fediverse" and separately to smaller niche communities they're interested in but sitting in their own walled gardens/bubbles. It's not necessairly a bad thing though - it is just different to what people are used to with social media like Reddit. It'll be a trade off - servers and communities have complete independence and some will go for what suits them - part of a big fediverse or only federating to smaller aligned communities.