Killer apps aren't always survivors. Consider how Visicalc fell to Lotus 1-2-3, and how Lotus 1-2-3 fell to Excel. Arguably, the killer app for NeXT was WorldWideWeb. While it's successors weren't developed in Objective-C, the prototype for the world wide web was.

Objective-C itself didn't have much of a chance for many reasons. One is that most APIs were based upon C or C++ at the time. The availability of Objective-C on other platforms will do little to improve productivity if the resulting program is essentially C with some Objective-C code that you developed from scratch yourself. Microsoft was, for various reasons, crushing almost everyone at the time. Even titans like DEC and Sun ended up falling. Having a 10x more productive API was not going to help if it reached less than 1/100th of the market. (Objective-C, in my opinion, was an interesting but not terribly unique language so it was the NeXT API that offered the productivity boost.) Also keep in mind that it took a huge marketing push for Java to survive, and being platform agnostic certainly helpted it. Seeming as Java was based upon similar principles, and a more conventional syntax, Objective-C was also less appealing.

I think the claim was not that the NS* API of NextStep was "10× more productive" but that the Objective-C programming language was. Objective-C is fantastic at calling existing C APIs. It's even easier than doing it in C# or LuaJIT, and much easier than doing it in Python, Perl, Tcl, Java, JS, etc.

You're right that there are programs that are just a thin layer of glue over existing C APIs, and the existing C API is going to largely determine how much effort that is. But there are other programs where calls to external APIs are only a small fraction of the code and effort. If OO was the huge productivity boost Jobs was claiming, you'd expect those programs to be much easier to write in Objective-C than in C. Since they made the choice to implement Objective-C as part of GCC, people could easily write them on other Unixes, too. Mostly they didn't.

My limited experience with Objective-C is that they are easier to write, just not to the extent Jobs claimed. OO makes Objective-C code more flexible and easier to test than code in C. It doesn't make it easier to design or debug. And, as you say, other languages were OO to a similar extent as Objective-C while similarly not sacrificing efficiency, such as C++ and (many years later) Java and C#.