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
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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.
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To be fair, I think such a fate in inevitable for most languages after many years of changes and development.
For this language to become default at Apple they had to be doing a massive amount of internal promotion - in other words they knew where it was going.
And then if that's the case, how were they not ready to solve the many problems that a big organization would run into? And all the schedule constraints that come with it?