- Fewer hardware manufacturers will publish drivers for other OSes.
- More computer manufacturers will sell computers with Windows preinstalled (and other preinstalled software that you will likely not want in your computer), making your computer more expensive and slower, and sometimes, less secure (search: eDellRoot).
- Fewer game publishers will publish cross-platform builds, even if they use cross-platform libraries. More publishers will prefer Windows-only libraries like DirectX instead of cross-platform alternatives like Vulkan.
- Fewer software vendors will create cross-platform software.
- Exposure to ads, forced updates, "telemetry" and analytics, and other forms of loss of control over YOUR computer.
- You will be rewarding Microsoft stockholders, including software villains from the 90s.
- If your country gets slammed with US sanctions, Microsoft can no longer sell you software (unless you decide to pirate it, be cut from updates, and be left vulnerable to Internet worms like WannaCry). Call it loss of cyber-sovereignty if you wish.
Now, about .NET... There are Linux builds of the .NET framework. You can use many IDEs to develop. But if you want an editors/IDEs, JetBrains Rider can be a good alternative to Visual Studio.