IIUC, it comes down to simplifying playback/subtitle rendering to the lowest common denominator among the various western streaming platforms.
The good/old subtitles in the ASS format required a more complex playback system than what Netflix/Hulu (and maybe blueray players) currently offer. This could be worked around by burning the subs into the video stream, but then you need to keep separate copies of your (large) video files for each subtitled language.
That doesn't seem like it'd be such a huge problem to me, but what do I know?
The post does a good job explaining the effective monopoly system at play that prevents real competition to provide any pressure to improve or maintain the prior quality.
It's an X*N*N problem: n_videos, bitrates, formats.
Assuming each video in its largest bitrate is... 2gb for example, and assuming S3 is $0.025/gb, that's a nickle per month or let's say $0.50/yr for that video.
Next up is reduced bitrates, assume you go from 2gb to 1gb and finally 500mb. Round up and you're at $1/video.
Now duplicate it to AV1 and MP4, and multiply that by English, French, and Spanish (oh, and let's say Japanese and Chinese too for good measure).
So a single 2gb video goes from $1/yr to $10/yr, and you're not doing "the dumb simple thing" for subtitles which would basically 4x your cost over "commodity subtitling services".
Or "simplify, simplify simplify", you reduce costs (cha-ching!), and become compatible for syndication or redistribution (cha-ching!)
... and they would have gotten away with it too if it weren't for those meddling kids!
S3 is >100x more expensive than hosting it yourself. You shouldn't argue about things being expensive using cloud prices.
Except ASS streams really aren't that big and don't have to be stored with each encode. They can just be in a separate file. And this is how cr used to serve them. Before they used hardware drm you could just download all of the separate sub tracks.
You don't need to multiply anything here, except the number of sub streams. One is ass, the other the primitive standards Netflix and other surges use.
Someone else was saying they maintained burn-in subs for devices that didn't handle ASS renderers. Even without accounting for the burn-in versions, using non-standard subs still bumps them off of commodity subtitling services and limits distribution/syndication.
Edit: and to the peer comment regarding S3 vs self host: regardless of 10x cloud cost, it's still 10x volume. Where 1TB local would do, now you need 10TB (10x the cost).
The labor of ass to ttml is there yeah. But the the factors are n_videos * languages * 2 Formats. And considering these are pretty compressible text(34MB->4MB for a completely bonkers sub track that includes animations, animated fonts and otherwise transformed text). I can't imagine that hosting costs cost more than their analysis.