Lattner probably left because Apple didn't give the team any breathing room to properly implement the language. It was "we must have this feature yesterday". A lot of Swift is the equivalent of Javascrip's "we have 10 days to implement and ship it":

https://youtu.be/ovYbgbrQ-v8?si=tAko6n88PmpWrzvO&t=1400

--- start quote ---

Swift has turned into a gigantic super complicated bag of special cases, special syntax, special stuff...

We had a ton of users, it had a ton of iternal technical debt... the whole team was behind, and instead of fixing the core, what the team did is they started adding all these special cases.

--- end quote ---