I don’t see too many jobs for nodejs developers, however, I do see a lot of c#/.net jobs, especially in my area.
Most of the nodejs jobs I see are sporadic in many different areas (wayy outside where I live too).
Are we going to see a decline in the use of nodejs on the backend in 2020 and beyond?
IMO, NodeJS is here to stay as long as JavaScript is here to stay - it’s just too useful for smaller projects and startups. One reason job postings might decline because small startups might be in position when they cannot afford hiring - it doesn’t mean less people use NodeJS 
Just consider following example, any person that utilizes server side rendering with React (create-react-app or NextJS) or Vue (NuxtJS) uses NodeJS on the backend (sometimes not even knowing that) and clearly SSR is on the rise
No
NodeJs fits a key niche for web application building, as its the only back-end stack that leverages JavaScript on the front and back-end. Stuff built on-top of those two key technologies might change (like frameworks) but those two key technologies (JavaScript and Node) wont go anywhere anytime soon.
C#/.net is popular as an enterprise language for a number of different applications (not only web development). Its possible most of the companies hiring in your area don’t build web apps, and thus don’t focus on JS as their language.
JS/NodeJs does not work well in some use-cases, such as high CPU applications, as such its not a be all end all language. The same is true for almost any language and technology in one aspect or another. This doesn’t mean JS/NodeJs isn’t in demand, it just means it isn’t going to be in demand for all use-cases.
Finally even “ancient” languages are still around in some form or another. Languages like C, COBOL, and Visual Basic have been replaced by newer languages and technologies, but one can still find plenty of jobs for these languages and technologies.
JavaScript, as the language of the web wont be going anywhere any time soon and still has a bright future.
The typical job req doesn’t ask for Node experience specifically, they just ask for Javascript and leave it up to the hiring managers to figure out if that’s server-side or not. It’s the same reason you see them usually asking for C# rather than Entity Framework or some other .NET tech.
And I wouldn’t lump C in with COBOL and VB. It’s still pretty much de rigueur in the embedded world and many other niches too large to really be called “niche”. It’s nice to see it slipping out of the mainstream though.
I agree that C isn’t in the same camp as COBOL. If you are close to the hardware or need portable performance, it’s very much still in active use.
C is one of the implementations for Python, by the way.
As is Python – a minor variant of a subset of python anyway. And it’s faster than CPython. GraalVM is a Java virtual machine that’s almost entirely written in Java, and also faster than the the HotSpot JVM (the standard JVM).
There’s a lot of benefits to implementing a language in itself, though almost all of them bootstrap from at least a tiny kernel of C or C++.
Bootstrapping always feels like witchcraft to me. “I use myself to make myself.” It’s a slick trick.
One of these days I really need to more deeply look into how bootstrapping works - too many cool things, too little time.