It's not that it's not big, it's that it is not on the same level than maps or gmail, IMO.
Before YouTube there was Google Video, which was moderately successful (it was just that YouTube was more successful, so it got acquired).
But there are just my opinions, of course.
See: https://mannhowie.com/youtube-valuation
Also see: https://www.cnbc.com/2021/07/28/youtube-is-a-proven-juggerna..., https://www.hollywoodreporter.com/business/digital/youtube-a...
I don't consider creating a product, and then retiring a product after many years a failure myself. Its more like a nuanced then, some producdts don't change with the times, and others dont work.
I did watch that TV series on how some people at google did some unethical things to incorporate the maps idea from a German company. Not sure how true that is, but if it is, then google is just like any other corrupt establishment, willing to do shady things at the expense of the original inventors.
Google3/Borg ecosystem should have been productized but that'll never happen now the way things are evolving.
One might argue that YouTube and Android should go in that list, but I disagree. YouTube was acquired and it never managed to become something really big (it failed to compete with Netflix for video streaming, with Spotify for music and with TikTok for social network). Android also, was acquired and without a pletora of hardware vendor supporting it, it would have failed too (I'd say that Android without Samsung is nothing -- of course, it's big on embedded things, like cars or TVs, but it's not a world-wide product like gmail).
And likewise it is failing in AI, against OpenAI, which looks like another engineering-first product.