This explanation and suggestions from @poVoq was the absolute best I've seen and I agree with nearly all of it.
The stated rationale for up and down-votes in Lemmy is to crowd-source content curation as an alternative to what other social media platforms do with their algorithms. A secondary goal is to crowd-source a sort of light moderation of comments by partially sorting comments threads after votes. This is similar to how it works in Reddit and HN etc.
While both sounds good on paper, in my experience neither works on Lemmy. I think this is mainly because the number of votes and comments (aka the number of active participants) is just too low on 95% of the posts for it to result in a meaningful content curation signal. But to make it worse, there are some notorious heavy users that abuse the system, which is easy to do as a few down-votes are usually sufficient.
I think the Lemmy devs still hope that at some point there will be sufficient scale for their vote curation system to work, but I started to think that a design that only works at a certain scale is fundamentally flawed in a federated context.
For Kbin I think it is even less likely to work, as the decision to use boosts as upvotes will make people hesitant to up-vote. While I agree that boosts make certain sense to use for this, I also know that people un-follow accounts that do too much boosting as it results in it occupying their entire home-feed. For me a boost is a stronger signal than a star/favorite and thus should be used more seldomly.
However, I still think using boosts as upvotes is not necessarily a bad idea, but it IMHO requires to get away from the up/down vote idea of Lemmy where it is encouraged to vote as much as possible (and even that isn't at a sufficient scale for the idea to work in Lemmy).
My suggestion is to lean into the idea to use boosts as up-votes and very explicitly use the "double-arrow" boost icon where currently the up-vote button is and do away with the down-vote there all together. That will integrate better with the rest of the fediverse and not lead to people being confused that others unfollow them over boost spam. You can still count the number of boosts to do some basic "hot" sorting, but this will work better with small communities that do not have sufficient scale.
You can still add a "dislike" or so button where currently the "comments, favorites, more" buttons are, but that should be an optional thing and not federate, or only federate in the local bubble (a concept of Akkoma for forming closer relations of a small number of instances).
Edit: for comments I would forget about votes all together, as on Lemmy vote abuse is even more common in the comments. Boosting comments out of context is often a bit jarring in the fediverse anyways, so the option to boost them should be probably de-emphasized.