We should allow older posts to receive upvotes

this is not going to happen, at least not in the near future. To support 3rd voting period, memory usage of node would have to increase even more (and even right now it is huge). Moreover, it is better for platform to support authors which writes regularly. If you find good post of some author, but you cannot vote for him, you can always follow him and upvote him as soon as he will write another good article.