With swift & obj-c your coding against the same library APIs, so in many ways it's a very similar level of programming. In many ways swift is far more complicated than Obj-C. With Obj-C iOS apps your not really writing that much C vs. the pseudo-smalltalk attachment to it.
We could of made it work. I think the one big benefit of the rewrite was how the architecture and nullable types basically gave us one more 9 of crash free rate, which we saw after the release with swift. With ObjC I could forsee how it would of crashed a bit more, because nullables aren't hard enforced in ObjC, but I could also see how we could of stepped around that by strong guarantees in the model & network service generation code.
It was a lot of work, but we had enough staff to make it someones job for half a year or a quarter, depending on how bad the migration is. With big codebases there are other issues that pop up with every new major Xcode release that we have to migrate, mitigate or fix. For example we are still at xcode 11 because of various bugs in xcode 12. Doing the migration was annoying, but it was not much compared to all the other issues, because the fixes were tractable.
As someone who was there, a rewrite was necessary, and with hindsight we should of done it with objective-c. The original obj-c app was built with under 10 iOS engineers in mind, and now Uber had 100s of mobile engineers work on one app in some form or another.
We didn't do a simple build scalability test until we were well into the project. If we did it would of revealed swift's build problems to us. Our swift experience is what slowed down our kotlin migration significantly. Today Uber android is still a majority Java app.
Even very recently the experience of working in swift compared to the old obj-c code base isn't as good. To this day there are debugger issues, xcode responsiveness issues and a slower build time.
Hiring on the other hand would be harder now, because it's getting hard to find people who know Objective-C and it's surface ugliness scares new people away. Swift is a decent language if you don't have to have a very large code base, so most people rightfully so don't have that much Objective-C, and if you were to work in Obj-C, you might as well go work at Facebook then.
With swift & obj-c your coding against the same library APIs, so in many ways it's a very similar level of programming. In many ways swift is far more complicated than Obj-C. With Obj-C iOS apps your not really writing that much C vs. the pseudo-smalltalk attachment to it.