I think maybe look at it the other way around. We should hope that one account at your preferred place (whether it’s kbin, mastodon or lemmy) be able to access everything. So if you have a mastodon account, you can already access content in the lemmyverse, albeit a bit clumsy. This is not possible from a lemmy account, afaik.
So once everything is united, you would be able to see content from anywhere in a format you enjoy through your one account