Even absent the problem with with older threads, there has to be a timeout, otherwise one iffy comment risks putting you into mute forever. That is, IMO, ridiculously punitive especially in light of the warning that you should not endorse if unsure, etc.
And keep in mind that an 'iffy' comment may not even be a bad one. It is pretty common on HN for more than one person to make the same basic point as someone else in a reply to a post (because they both started posting around the same time and didn't see the other replies before making their own) but due to either timing and/or karma boost pulling one person to the top, the rest are basically ignored for upvoting -- I can't imagine that situation would be any different for endorsing. Who wants to endorse a post that says the same thing as another post which is already endorsed (but just happened to be posted 30 seconds later, or by someone with less of a karma boost?).
And keep in mind that an 'iffy' comment may not even be a bad one. It is pretty common on HN for more than one person to make the same basic point as someone else in a reply to a post (because they both started posting around the same time and didn't see the other replies before making their own) but due to either timing and/or karma boost pulling one person to the top, the rest are basically ignored for upvoting -- I can't imagine that situation would be any different for endorsing. Who wants to endorse a post that says the same thing as another post which is already endorsed (but just happened to be posted 30 seconds later, or by someone with less of a karma boost?).