I'm working on the frontend for it rather than the backend so I'll comment more about that
But a new project allows for way easier change of the base aspects. For example im currently working on a theme system thats allows for dynamic themes created at runtime as opposed to it needing to be built in. Also a components library. If this was added onto lemmy ui it would involve massacring the current structure of the UI to essentially make it a new project anyways
Originally was working on the stuff in a new UI on my own but I've merged that into what's happening with sublinks since they're making a new UI anyways as well and would let more of my UI changes to get connected up to the backend easily and shared across multiple frontends
In terms of technologies it also allows the federation code to be completely separated out from the api. Federation is currently its own project so it can be scaled separately and its made in go
Also allows for more organizational changes since we have more control over how the project is structured and the structure of how we talk to each other and decide on changes is different than how its done with lemmy (having a matrix space we talk to each other and there being weekly meetings as well)
Moderation tools is the first milestone after parity but theres also other milestones as well in terms of changes made that differentiates it from lemmy visible on our task board thats public on the github repo
Normal thats theres going to be multiple of the same type of software as people have different goals of what it should be and how it should be organized. Bevy and godot both exist in the open source gamedev space. Theres 7 misskey forks that all mostly aim to do different things but share the misskey api (and a lot of them also use the mastodon api). One of which (iceshrimp) is currently having a rewrite to change the tech stack and make it easier for them to add features
Nothing would change about the community itself if it goes from lemmy to sublinks. Still accessible on the federation as normal and on version 0.1 the core features should have parity
Reposting my comment I did before:
For world Ruud commented about that before and nothings been decided currently on theyre going to handle it (I assume youll see some sort of post in their meta community way before anything happens)