Can I as a programmer depend on elementary OS Loki instead of Windows 10? Should I uninstall Windows 10 and use elementary OS Loki as my default OS for web & application programming?
It depends on the tools you rely on and their dependencies. Elementary OS should be fine for web development as the tools you need will only be a few installs away. You also don't need to worry much about compatibility with Microsoft's native browsers like Edge and IE as Microsoft is generous enough to provide minimal VirtualBox images for free without the need for rebooting to a full Windows installation just to test a website.
If you're using Node.js or similar cross-platform tools to build your applications, you can probably forgo Windows.
If you need to develop games or applications that needs to be also available natively on Windows, I suggest keeping the Windows 10 for compiling and testing your builds.