The EdgeHTML rendering engine, fork of Trident, fork of Spyglass Mosaic, fork of Mosaic will be replaced by the Blink rendering engine, fork of WebKit, fork of KHTML, fork of Khtmlw. Existence of standards and minority implementations does not prevent emergence of monoculture. Microsoft knows this effect very well. Contributing to Chromium rather than developing proprietary component of Windows will allow Microsoft to deliver its proposed standards to majority of developers instead of minority.
The web browser consists of two major parts – UI (Chrome, Firefox, Opera, IE) and engine (Blink, Gecko, Presto, Trident). One part exists independently to the other. In case of Edge, the UI is a UWP app and the engine is edgehtml.dll written in C++.
One could say that Microsoft is abandoning its own browser because nobody is using it. It simply does not make sense because the success of the UI can be measured by counting of people using it while the engine success can be measured by counting of standards it implements. Now the engine ends and the UI remain because nobody uses the UI – it’s nonsense.
Developers are optimizing their (web) apps for Chrome because they are using its DevTools. IE/Edge F12 Developer Tools have never established as an industry standard. It wasn’t in Microsoft’s interest which rather invested into Visual Studio HTML & JavaScript debugging tools.
Today’s Microsoft position is different from the position which was 20 years ago.
Microsoft in 1998
Windows 98 was the first Windows version with contained HTML and DVD support out of the box. It allowed running of SQL Server 7.0 for reliable business data storage and Office 97 for productive management. Microsoft sells Windows operating system to PC makers to unlock the hardware potential and to businesses as a development platform. When you sell a platform, you are competing with functions it supports.
Microsoft in 2018
Microsoft is selling its own hardware (Surface and Xbox product lines), renting its own servers and providing services (Azure and Office 365), selling advertisements (Bing Ads) and charges provisions from apps distribution (Store app). Windows sells have almost evaporated and added value of the in-house rendering engine is gone. When you sell devices, you are competing with user experience.
Adopting Blink will allow Microsoft to focus on web standards (this is where the innovation happens) and performant Windows implementation will be delivered to majority of web browser users on Windows.
Most of web developers decided that one open source reference implementation model is better than many independent implementations rising from one standard. So be it.