It's easier to read and navigate a well-written Swift codebase than a well-written Objective-C codebase

Conversely, it's easier to debug an Objective-C app than a Swift app, simply because compiling and debugging is so much faster, and debugging so much more reliable

I don't know about a software quality drop being attributable to the migration to Swift. So many other things have also happened in that time — much more software that Apple produces is heavily reliant on network services, which they are not very good at. I find Apple's local-first software to be excellent (Final Cut Pro X, Logic, Keynote) and their network-first software is hit-or-miss

They have also saddled developers with a ton of APIs for working with their online services. Try to write correct and resilient application code that deals with files in iCloud? It's harder than it was to write an application that dealt with only local files a decade ago!

Swift is easy to blame, but I don't think it's responsible for poor software. Complexity is responsible for poor software, and we have so much more of that now days