As a web dev these days I feel like we have it the easiest:
If you want to do mobile, try making it a PWA. If it needs to be in the app store (which these days I often see portrayed as more of a necessary evil than something desirable) go hybrid. If hybrid is too slow, try React Native or NativeScript. All of these can build on your existing knowledge of web technologies.
If you want a desktop app, you probably don't actually want a desktop app anyway, so just build a decent web app. If you need something that runs outside the browser, use Electron. Or React Windows if you only target the UWP.
If you want to build something on the server-side, there's of course Node.js. If you want something more tightly integrated with the database try ArangoDB. Again, same language you already know.
There are still niches where you need the raw performance of specialised programs written in other languages than JS, of course, but they are growing smaller. It's basic economics: if you can hire two full-stack JS devs instead of one iOS, Android, Windows and back-end dev each, you can cover the same technologies with half the salaries. This is especially true if the specialists end up doing mostly busy work because you don't actually need their specialised expertise all the time.
Few companies can afford to have all these dedicated specialists on the team but they may still want to cover multiple markets (e.g. an app for desktop and Android). In this case going for the web is the obvious choice for the smaller budget. The alternative would be hiring multiple contractors or outsourcing the project to a company that has the resources to provide all of the needed specialists all the time.