Licensing of Microsoft products has historically been a minefield of complexity. You can even sit for an official exam on it and get a qualification. Microsoft's recent move toward open source practices is very encouraging, as the biggest benefit of open source is not the free monetary cost but that you don't have to think about licensing costs. You can also fix issues, and with a permissive license (such as MIT), you don't have to worry about much. The time costs and cognitive load of working out licensing implications now and in future can dwarf the financial sums involved (especially for a small company or startup).
Despite the new .NET Framework being open source, many of the tools are not. Some editions of Visual Studio and SQL Server can be very expensive. With the new licensing practice of subscriptions, you will lose access if you stop paying, and you are required to sign in to develop. Previously, you could keep using existing versions licensed from a Microsoft...