Yup, that's what Lemmy does: https://join-lemmy.org/
https://join-lemmy.org/instances says there's only 2.6k monthly active users in the entire "lemmyverse". That's not even getting off the starting blocks for even a single niche sub-Reddit.
Getting people to even humour the idea of going elsewhere when there's genuine value in pre-existing content is difficult, especially when it comes to troubleshooting, guides, advice et al, which unfortunately, Reddit is brimming with.
Windows Phone was a pretty good example of the "no users" problem: a solid alternative to iOS and Android, but not much in the way of big apps because of a lack of users, but the lack of users primarily stemmed from a lack of big apps. Chicken-and-egg scenario ad infinitum.
Not sure if all data is available anywhere? (Common crawl? Those big internet scrapes used to train language models?)
Most of the value that I derive from older content is being able to recontextualize it for the present day, especially with extremely niche stuff.
However, user hostility from Reddit itself is likely going to lead to an exodus of power users, just like with Twitter post-acquisition. It might not be huge numbers of people quitting, but they're meaningful users, which are a sad loss community-wise. Especially if you assume that way more people just view a platform than actually post things on it, which is apparently vastly skewed in that way for most platforms; "lurkers".
[1] https://academictorrents.com/details/7c0645c94321311bb05bd87...
(I also imagine something like >95% of all Reddit content was produced in-between 2012 and 2023.)