You don't have to "rewrite the app". You run the migrator, then fix up what it didn't migrate.
Swift 3 was notorious for having migrator issues, largely due to the fact that it was a pretty radical change and a huge number of method names were changed.
Personally, on our hybrid app, the migrator did about 1/3rd of the work, and I did the rest, without any other engineers' involvement. Overall it took maybe 3 days to migrate the whole app.
It was a way of talking. I said it that way because the migration to swift and from swift 1 to swift 2 we had the double of developers for iOs than Android and still have to wait for them because of so many issues they were having.
Seems like things are not that bad now. But stills scares me that with every new version you lose a developer for some days (or weeks).
The whole 1.x to 2.0 stretch was a rough ride. I had to maintain software without From 2 to 3 was pretty OK since very little changed in the language only API's got updated. From 3 to 4 mainly adds stuff and doesn't break much.
It really doesn't take all that long. I've migrated apps consisting of 20K-30K lines of code in a couple hours–mind you, many of the changes were not due to Swift 4, but those in the Cocoa/Cocoa Touch Swift overlays.
I said to swift, I meant from objective-c to swift. And also from swift 1 to swift 2, where problems weren't only the language but the apis an integration with the old objective-c library. I doubt it takes a couple of hours do that stuff (hour project was close to 100k lines, to one day for you), but If it really takes you that short time companies are going to fight hard to hire you!
I remember all the time the iOs developer spent with every iOs release. He didn't look any happy