The "WAV" format is not outdated and is a primary format for saving audio files in most audio editing applications for obvious reasons (AIFF is Apple's version of essentially the same format)
Outdated as in "It's old"
Doesn't mean is a insult or bad thing. Just used the term to describe how old it is. Of course at the time, it was very essential to anyone using cd format. Games on sega cd, or even pc engine cd was using wav format for the high quality stereo audio they use. While it wasn't a terrible thing, I was more of a fan of the FM synthesized samples that the console chips provided. Mostly cause the the music in the game sounded like it belongs with the game and loops instead of stopping and starting again like audio files did.
Besides the games being on cd, it didn't mean much because each game did whatever it wanted with the disc sized storage. Some games files sizes was dominated more on wav audio recordings, some games focused on low quality video recordings. (And if you're familiar with those games, they weren't the most popular of the sega cd) Just feel like the high file size of these games wasn't because the games was much bigger or better than the cartridge counterparts.
Sonic cd is about the same length as sonic 1. Game uses both low quality video files for animations and high quality wav files for music of stages present and two different futures. Of course the music sounds better because of the wav files. The game could instead had more stages or even longer ones. But the big file size is all from audio, so much so the past stages was stuck using the audio chip for music. Obviously, it's inconsequential about the file size because it's on the disc, that's the point. But for emulation, it can be a big burden for storage devices if you don't use original disc. Which is why the MP3 or OGG compatible emulators is nice. I won't know the difference but I'm sure any audiophile would.
Still, if the only point of introducing the sega cd was for pre recorded music, voice overs, or video files... What does it do for the games? Pier Solar and the great Architects came out in 2010 for sega genesis. But it did do something unique like use a 64 Megabit (8MB) cart for the game and sega cd for "Enhanced audio" (Technically it was PCM from sega cd sound chip, not streamed wav files) But the only reason I mention this is cause is interesting. Sega CD could have just been used in tandem with genesis carts to provide video and audio cutscenes much like Snes homebrew MSU-1.
I'm sure it would have been expensive to produce cart and disc for the games, but it actually did that. Sega CD FMV games came out had required 32X for improved graphics. Neither of them can work without the genesis anyway. So why not make a genesis game that gets enhancements from sega cd? A game that doesn't require the add-on to play but would provide optional high quality soundtrack and video cutscenes or even voice overs if it did. (DLC ANYONE) Unless a developer wanted to make a much bigger game, could it have been sega cd exclusive. (Or maybe use LOCK-ON TECHNOLOGY)