A lean language reduces the surface area for beautiful expressiveness by clever people - making sure the dumb/junior guy who is maintaining your project in the future can actually fully understand what is written. And it can build and run fast - so you can iterate and test out software behaviors fast.
No one in this world is immortal. If it takes too much time to grok code, write/compile/run tests - a developer will be disincentivized to do so, no matter how amazing the language features are.
My guess is that Swift has adversely affected Apple's overall software quality. As more software moved from Objective-C to Swift, quality has dropped precipitously.