Just because you don't care about the downsides does not mean there are no there. First of all, which subset of C++ should be used? It is a huge language that only very few people can claim to master. So lot's of people wouldn't be able to understand or change the code anymore. That is quite the significant downside.
Not to mention all the other downsides mentioned in my other post, like being slower to compile, bigger binary and so on. Yes, those downsides might not matter to you and should not matter for many projects but they do exists.
C programmer often care more about building the best possible software not so much about having the best possible development experience while building.
That said, Rust has much better safety guarantees, so it DOES offer some significant upside that C can't.For more complex software it is is definitely a very good choice.
I do have experience using both C and Rust. Arguably, my C++ is very limited but things like C++ being a relatively large language are well established facts and quite uncontroversial.
It is true that some people resist using anything introduced since they first learned, with a few mired in C. But there is never any need to "agree" on that.
"Bigger binary" is scurrilous propaganda. Compilers nowadays use the same code generator for C and C++: say the same thing, get the same code. Furthermore, optimizers have to guess less at you are trying to do, in C++, so can do a better job. C++ code for the same task is often smaller.
"C programmers often care more about building the best possible software": this is just crude slander.
Finally: "Rust has much better safety guarantees": C++ offers much the same guarantees, where you choose to exercise them.
It is based on a stable standard.
It is extremely portable and not in a target major platforms way - that is much better done in other languages - but in a being able to run on nearly every OS and hardware in existence. You can feasible port your C software to you weird hobby operating system.
It is simple enough that you could write your own compiler or at least something to bootstrap a proper one.
It is the Latin of programming languages. It is the common, cultural core we have. Nearly all bigger languages have some way to interact with C. If you want to write a library that can be easily used from any language, C is the way to go.
It is (relatively) fast too compile and creates binaries of minimal size that run at maximal performance. (Yes, you do make trade-offs in some of these aspects when using Rust or C++. Those trade-offs are worth it in many cases but let's don't pretend they don't exist. C serves as a performance gold standard for a reason.)
Yes, it is also a horrible language is many ways and should not be used for most projecs but the advantages can make all the difference for certain use cases. For projects like SQLite or Lua it was for sure the right choice to use C.