Uber does offer an API, but I think part of the problem is that a lot of these providers either worry about becoming interchangeable, or believe - rightly or wrongly - that their brand is strong enough not to want to let intermediaries cut in.
So I think you're right in as much as that at least in some areas it'll take providers big enough to say "if you don't work with us all of our millions of customers will see your competitors first".
A smaller player will likely have alto start small and be content to not have as broad coverage as they'll like.
Maybe couple with papering over some with human intervention like more traditional concierge services until they can go to providers with proven demand.
Something that was very popular back in the day, and was a great filter for companies, and something that most of the current giants did to their competitors, before declaring via laws that nobody can do it onto them to secure their market position indefinitely.
In regards to the last sentence, 100% - I think this will eventually have to come from one of the big tech
If I could choose I’d much rather book by sending a calendar than a text message
To their credit, clicking on a restaurant and seeing available ordering platform options with fee info is nice.
Similar for social things. I need to coordinate movie tickets, dinner reservations, or tennis courts not just with the supplier but with my friends.
A generalized text interface for consumers would have to be not only as good, but noticeably better in order to create some monetizable advantage.
If this arises, I’d expect it to come from an existing body of users (Siri/Alexa) encouraging suppliers (Uber, OpenTable, DoorDash, movie theaters) to build towards a common API. Without that pump priming with tens of thousands of local users already on the platform with money to spend, it’ll be hard to gain traction I think.