But you need to differentiate between downvoting a post, and an upvote being invalidated. Accounts get deleted all the time for various reasons (e.g. self-deletions), and when those accounts have not made "a lot of impact", their votes are invalidated.
I don't know if this is the case here, but that seems as the most likely scenario.
While I don't actually disagree with the sentiment in your question, I think an answer that points out what I see as the problems with your feature allows people to discuss better. So this is somewhat devil's advocacy and not outright opposition :)
For your point a), I think there are times when you might answer a question without thinking it's worth an ...
I am adding this answer because the question came up again recently and, while much of what I wrote is a little redundant, there is an aspect of the culture of MSE which I think is worth discussing.
In general, a vote on the main site indicates that a post is poorly researched or of low quality. On Meta, votes should, for the most part, be interpreted ...