It boils down to the self discipline of individual developers. If you’re meticulous in making sure you’ve dotted your I’s and crossed your T’s and haven’t let too many of those insidious cast statements slip in, you can write reasonably safe Obj-C, but during my career as an iOS dev I have yet to encounter an Obj-C codebase that was fully written by such developers. They’ve all been riddled with gratuitous casting, erroneous type assumptions, and other bad practices (that aren’t possible in Swift) to the point that it’s a wonder that they ever functioned as intended.

Personally though, I’ve found that even when I’m keeping a close guard on things when writing Obj-C and am sure to avoid all the obvious pitfalls, bugs slip through in the most unexpected places, so I’m quite happy to have the Swift compiler double checking everything for me.