The Accidental Tech podcast had a long interview with Lattner about Swift in 2017 [0]. He makes it out as something that had started as side-project / exploration thing without much of an agenda, which grew mostly because of how good positive feedback the project had got from other developers. He had recently left Apple back then, and supposedly left the future of Swift in other peoples' hands.

[0] https://atp.fm/205-chris-lattner-interview-transcript#swiftc...