pjmlp has it right, but I'll add some further clarifications.
One of the most important aspects of Microsoft's success is network effect. People use Windows and Office because other people use Windows and Office.
But network effect also works on the development/platform side. Devs write apps in the technologies they know and are comfortable with, and those tend to be the technologies they've used for personal or work projects. Today, far fewer of the top tier (even the top 50% really) of developers are using Microsoft technologies, or are even familiar with them. The "cool kids" code in Ruby or Python or Scala or Golang, or mobile specific languages. If they want to get things done to pay the bills they might fall back on java or PHP. But the idea of using C#, .NET, or coding Windows Apps? Far fewer devs under 30 are doing that than they were 10 or 20 years ago.
With fewer devs coding on your platform that means there will be fewer apps, especially the most desired ones, and eventually that will impact the user base as well and weaken the entire business. The era where nobody cares whether a computer runs windows is on the horizon today, even though it may be many years away. If you look at the big apps and systems from the last 5 years you see that a lot of them are either web based or mobile (specifically android/iOS as well).
Microsoft used to understand this and it was a major reason why they invested in Visual Studio for all those years when it was unprofitable. But they've had a period of unrivaled market dominance which has only recently been challenged, so they lost their way for a while.
Catering to devs is how you maintain market relevancy with your platform, which is something Microsoft desperately needs.
Additionally, improving the relationship with the dev community and attracting more devs to your platform makes hiring easier (especially hiring talented folks). It's almost impossible to objectively quantify the benefit of that but it's an enormous factor. Microsoft has been bleeding talent for years and has been starved of a lot of talent for a long time because they were perceived as being old and boring and requiring a completely different set of proficiencies to work there. If Microsoft wanted to they could turn that around and start paving a road back to relevancy in the dev community again.