That's only obvious in hindsight. Back in the day, people said how in the world am I going to use the same search box for buying a car (color, make, model, photos) as I do for finding the latest concerts in my area (map, seating charts, etc.)

And you're right vertical SaaS DID become a thing, and so will vertical AI, but the horizontal versions of both (search and SaaS) crush the vertical ones (Google for search Microsoft for SaaS), and I believe it will be the same. Theres a layer above what you are talking about (e.g. Teams as the product vs Slack as the company).

Horizontal has a higher TAM and Vertical is easier to execute.

But this is besides the point. My point was that productivity is a minority of the TAM.