I'd wager there's 95% of the benefit for 0.1% of the CPU cycles just by having a "search transcript for term" feature, since in most of those cases I've already got a clear agenda for what kind of information I'm seeking.
Many years ago I make a little proof-of-concept for displaying the transcript (closed captions) of a YouTube video as text, and highlighting a word would navigate to that timestamp and vice-versa. Such a thing might be valuable as a browser extension, now that I think of it.
YouTube already supports that natively these days, although it's kind of hidden (and knowing Google, it might very well randomly disappear one day). Open the description of the video, scroll down and click "show transcript".
Searching the transcript has the problem of missing synonyms. This can be solved by the one undeniably useful type of AI: embedding vector search. Embeddings for each line of the transcript can be calculated in advance and compared with the embeddings of the user's search. These models need only a few hundred million parameters for good results.
Many years ago I make a little proof-of-concept for displaying the transcript (closed captions) of a YouTube video as text, and highlighting a word would navigate to that timestamp and vice-versa. Such a thing might be valuable as a browser extension, now that I think of it.