> use this for channels that don’t offer traditional podcasts

Yes. Plenty of such channels out there.

> saving the files would get costly after a while

You don't have the save the audio files. Its possible to just directly link, within the RSS feed, to the audio files hosted on youtube servers. The server simply has to periodically generate the RSS feed using YouTube api calls, and then host the RSS files. Surely, storing even 10s of thousands of RSS feeds won't cost that much. They aren't also being requested that often, so bandwidth costs are minimal.

> Would you pay for something like this?

Sure.

Does youtube host an audio only version? I wasn't aware that was something that they did. My understanding is that all Youtube content is video only. Even if the screen is blank, you're being served some content with a video codec. I've done a little cursory searching and to my knowledge, Youtube does not offer an MP3 link to any content (from their API or embedded on the page).

They offer a mix of audio-only, video-only, and video+audio. Here is a list for a particular video (great music btw)

    > youtube-dl -F https://www.youtube.com/watch?v=GMiaVo_x1EY
    [youtube] Extracting URL: https://www.youtube.com/watch?v=GMiaVo_x1EY
    [youtube] GMiaVo_x1EY: Downloading webpage
    [youtube] GMiaVo_x1EY: Downloading android player API JSON
    [info] Available formats for GMiaVo_x1EY:
    ID  EXT   RESOLUTION FPS CH │   FILESIZE  TBR PROTO │ VCODEC       VBR ACODEC      ABR ASR MORE INFO
    ──────────────────────────────────────────────────────────────────────────────────────────────────────────────
    sb2 mhtml 48x27        0    │                 mhtml │ images                               storyboard
    sb1 mhtml 80x45        0    │                 mhtml │ images                               storyboard
    sb0 mhtml 160x90       0    │                 mhtml │ images                               storyboard
    599 m4a   audio only      2 │   13.42MiB  31k dash  │ audio only       mp4a.40.5   31k 22k ultralow, m4a_dash
    600 webm  audio only      2 │   16.34MiB  37k dash  │ audio only       opus        37k 48k ultralow, webm_dash
    139 m4a   audio only      2 │   21.27MiB  49k dash  │ audio only       mp4a.40.5   49k 22k low, m4a_dash
    249 webm  audio only      2 │   23.83MiB  55k dash  │ audio only       opus        55k 48k low, webm_dash
    250 webm  audio only      2 │   30.99MiB  71k dash  │ audio only       opus        71k 48k low, webm_dash
    140 m4a   audio only      2 │   56.45MiB 129k dash  │ audio only       mp4a.40.2  129k 44k medium, m4a_dash
    251 webm  audio only      2 │   65.21MiB 150k dash  │ audio only       opus       150k 48k medium, webm_dash
    17  3gp   176x144      6  1 │   21.50MiB  49k https │ mp4v.20.3    49k mp4a.40.2    0k 22k 144p
    597 mp4   256x144     13    │    4.97MiB  11k dash  │ avc1.4d400b  11k video only          144p, mp4_dash
    598 webm  256x144     13    │    6.97MiB  16k dash  │ vp9          16k video only          144p, webm_dash
    160 mp4   256x144     25    │    5.51MiB  13k dash  │ avc1.4d400c  13k video only          144p, mp4_dash
    278 webm  256x144     25    │   15.44MiB  35k dash  │ vp9          35k video only          144p, webm_dash
    133 mp4   426x240     25    │    9.53MiB  22k dash  │ avc1.4d4015  22k video only          240p, mp4_dash
    242 webm  426x240     25    │   28.68MiB  66k dash  │ vp9          66k video only          240p, webm_dash
    134 mp4   640x360     25    │   15.47MiB  35k dash  │ avc1.4d401e  35k video only          360p, mp4_dash
    18  mp4   640x360     25  2 │ ~ 73.46MiB 165k https │ avc1.42001E 165k mp4a.40.2    0k 44k 360p
    243 webm  640x360     25    │   55.53MiB 127k dash  │ vp9         127k video only          360p, webm_dash
    135 mp4   854x480     25    │   22.52MiB  52k dash  │ avc1.4d401e  52k video only          480p, mp4_dash
    244 webm  854x480     25    │   89.33MiB 205k dash  │ vp9         205k video only          480p, webm_dash
    22  mp4   1280x720    25  2 │ ~ 90.62MiB 203k https │ avc1.64001F 203k mp4a.40.2    0k 44k 720p
    136 mp4   1280x720    25    │   32.23MiB  74k dash  │ avc1.4d401f  74k video only          720p, mp4_dash
    247 webm  1280x720    25    │  166.48MiB 382k dash  │ vp9         382k video only          720p, webm_dash
    137 mp4   1920x1080   25    │  106.40MiB 244k dash  │ avc1.640028 244k video only          1080p, mp4_dash
    248 webm  1920x1080   25    │  271.78MiB 623k dash  │ vp9         623k video only          1080p, webm_dash