Obviously StackOverflow and TIOBE examine different things. TIOBE counts what is being used, while SO counts what programmers would like to use? Or they ask different people?
It's a yearly survey that any user can fill in; the overall methodology is explained here (
link). I don't see the questions there anymore but they can be downloaded along with the data (
link). There are questions like:
Which <b>programming, scripting, and markup languages</b> have you done extensive development work in over the past year, and which do you want to work in over the next year?
There was also a blog entry a few days ago which details the programming languages (
link), that I haven't read yet but which looks interesting.
I've seen other stats like Languish (
link), but I'm sceptical because it's compiled from questions on SO and problem reports on GitHub. I can't help thinking the most problematic languages will have the upper hand.
One question (since you've mentioned both of them): Rust or Kotlin?
Are you asking which one you should try first? If I had to choose between Kotlin and Rust, I'd say it depends on the type of development.
Rust is great for many things, it's very fast, very safe and has a good, simple and unique build system (no bash/Makefile/CMake headaches) that also gives access to all the libraries published by others, you just need to say what to include. But it's harder to master and you must get used to a few unique paradigms rather than trying to translate what you are used to in C++. It takes more time to write and usually more lines of code than Kotlin. The compiler is extremely helpful - oftens tells you how to fix problems.
Kotlin is a sort of C# running on the JVM. It's IMO the best OO procedural language (with many functional programming features) in terms of clean code and features that I know of. It's very easy to learn, and very efficient. Kotlin is great for UI, that's probably why it's been endorsed by Google for Android apps. But it's JVM and uses a garbage collector, it won't be as performant as C++.
Note: it can also compile to native and JS, but then you lose the Java standard library.
C++ is a sure value IMO, I doubt it will be replaced anytime soon no matter what some claim about Go, Rust or Carbon. And I'd definitely choose C++ over Rust for everything UI-related, because the frameworks are more mature, then because there is no inheritance in Rust and that's where it's most needed. There's dynamic binding, traits, generics, closures and other niceties though, it's definitely not low level.
TLDR: Kotlin is great, but it will be redundant with what you already know. You'll be more rewarded with Rust, I think. Which seems to confirm your conclusion.