Look at Eero for to how a simple updated syntax could look. On top of that, many of Swift's features could run on an updated ObjC as well. The problem is that the features that required a whole new runtime and compiler model are very few.

In other words, we could have gotten almost-Swift by building squarely on ObjC and would have avoided most of the runtime and performance issues that still plague the language. Plus it would have been a simpler language than Swift is.