Actually, in a perfect world I would rather expect there to be a perfect type representing intervals in Swift which we can use everywhere and which support everything we need, including days, months and years in all calendar systems we like.
But, in our everyday work we are using UIKit which has many APIs which expect a TimeInterval. If I didn't see the DispatchTimeInterval type from Apple, I actually wouldn't have even considered to pitch this change to Swift. But we have two time interval types in Foundation, one of it has explicit units, the other doesn't. And the one that doesn't is part of the more often used APIs in UIKit. I feel like we should at least harmonize these types.
Maybe we don't need it in 15 years when Apple rewrites its APIs, but we need it in the meantime and millions of projects include implicit units. I feel like that's worth this rather small change.