The post edit ability after one week has already been addressed. Posts can now be edited after that time period, though the blockchain retains all the edit history so people can always see the original content if they choose to. Some front end sites (such as Steemit) simply haven’t changed the editing user interface to reflect this as it is a recent change and not all of the witness servers have upgraded yet.
Voting ability is another matter. There are serious processing & technical issues that keep all content from being perpetually votable. I think the best workaround is to engage the author with a good comment. If they reply, you can upvote the fresh reply instead of the expired post to reward the author & content.
RE: Suggestion For Steemit - Ability to edit posts and upvote, comment immutably.