my opinion: the knowledge is not going to "lost", cause niche community probably will crawl their own data or reddit wiki/FAQs before they retreat to other platform, lemmy or not. However, that also means you can't just google the phrase and put site:reddit in the search term to quickly filter your results.
Yes it sucks that say in the future you want to search for something it would take more time to reach the same result or user base, it will still exist somewhere. Like those emulator or homebrew communities that are universally chased/DMCAed around the internet, they still exist and you just need to spend more time to find them.