Just to give them the benefit of the doubt, many video sites do this (YT being the biggest). It allows you to change the video source on the fly (changing the bitrate for example) without effecting the audio playback so preventing any audio glitches during the swap. People are a lot more forgiving of video artifacts then they are of audio glitches. It also allows you to have a lower bitrate copy of the original video for mobile playback without having duplicate copies of the audio.
So maybe thats why they do it and not some evil plan to annoy you from saving the video (Tools like youtube-dl will happily merge the video and audio into a single container after downloading both pieces of the puzzle).
Although, I don't use reddit anymore, I gave it up a few years ago, so maybe they have gone completely evil and the video/video track splitting is simply to annoy hotlinkers (I know they have been throwing more and more dark patterns in to their redesign, the redesign was basically the kick I needed to quit that habbit). Just giving them the benefit of the doubt on that one for now as there are legit reasons to do it.
EDIT: 1) My ref to youtube-dl is about how it will merge the 2 after giving it a single YT URL. No idea if it works on reddit, but as it works on a number of sites I wouldn't be surprised if it also works on reddit (its been years since I've been on reddit in any meaningful way, so never had the urge to try).
2) ffmpeg and avconv will merge pretty much any audio track with any video track you throw at it into a single container without the need re-encode the video. In fact iirc I think those are the tools youtube-dl will use to merge the tracks after downloading content from YT.
3) as for annoying hotlinkers, there are (imo) better ways to annoy hotlinkers then spliting the audio/video tracks. Personally my fav way is to replace the video being served to the client with a copy of Never Gonna Give You Up serverside after the hotlinking is detected - but serving the orig video when its not hotlinked, but I'm a dick :-P (a more sane and polite way would be to simpy not serve any video at all, saves wasting bandwidth).
I guess it's a bit of both. A technically better solution that also helps your marketing is simply a win-win-situation for them.
But it's kinda ironic that the upgraded the user experience with better playback while similarly degrading it by disallowing hotlinking. Typical sidegrade.
On the video I used I saw 5 video encodes (96, 240, 360, 480, and 720 - it could be that the orig video was 720 so there is no point encoding a 1080 video). So yeah, it saves then have atleast 4 dupicates of the audio if each video was in its own container also containing audio when you can mix it client side.
I also put the reddit URL into youtube-dl which downloaded the two pieces and murged them just fine.
So for me personally I'm still going to give them the benefit of the doubt on that one as its a pretty common way to do streaming video these days. But I see your point that it came with an added benefit to them that it makes it more difficult to hotlink to the video, but its still easy enough to download and rehost elsewhere.
On something, anyway. Whenever Reddit tricks me into opting into the redesign I'm disoriented by just how much of reddit I cannot access. Following comment chains is a chore, getting all comments to a post impossible, instead every page is a duplicate of the front page with random barely related posts getting shoved in my face I already didn't care about the last three times reddit wanted me to.
Is the only metric developers are judged by the number of (intentional or even just accidental) top-post clicks per user?
Sounds like they are optimizing for a metric, rather than for a goal.