Honest question: who in this field has a better track record of their visionary ideas leading to real-world implementations than Alan Kay?
Not everything ended up being adopted in the form he envisioned or with the semantics he proposed, but there have been a lot of right calls and influential designs in his 50+ year career.
What kind of impact did Engelbart have after "the mother of all demos"? Much more limited than Kay, IMO. Kay’s Dynabook is arguably just as important, and he went on to do a lot of other stuff.
Same for Berners-Lee. Sure, he remains influential over the web's incremental progress on W3C, but anything more visionary seems to be a miss: XHTML, semantic web, the Solid project...
The mother of all demos is the world we live in now. He demonstrated a working model of 30 years into the future. It's like someone walking on stage and showing you what life will be in 2050.
Engelbart invented the mouse and the entire idea of pointing at things on the screen to interact with them. The results of that interaction is now what you call "the web" (hypertext).
Englebart also substantially inspired Kay, but each of these people would probably have not had success without the network of ispirations. In fact I think we should talk about it like this; Bush->Englebart->Kay->Berners-Lee (superficially) rather than the individual on their own.
It's also interesting that none of these people were at the head of big successful companies, though not surprising since that basically involves a lot of compromises.
Not everything ended up being adopted in the form he envisioned or with the semantics he proposed, but there have been a lot of right calls and influential designs in his 50+ year career.