The upvote button is all that is needed.
The upvote is useful, because it’s a very concise way of expressing support or thanks without needing to post about it.
The downvote isn’t useful on its own, because it would be very rude to give somebody a downvote without also posting a polite explanation about their differing opinion. But having the downvote button makes it too easy for somebody to rudely post a downvote without saying why.
We’ve survived for many years without having a downvote button and not had any problems due to it. Best keep it that way 🙂