Backwards compatibility, security, edge cases, downstream effects on other libraries that are reliant on LINQ, etc.
One guy with an optional library can break things. If the .NET team breaks things in LINQ, it's going to be a bad, bad time for a lot of people.
I think Evan You's approach with Vue is really interesting. Effectively, they have set up a build pipeline that includes testing major downstream projects as well for compatibility. This means that when the Vue team build something like "Vapor Mode" for 3.6, they've already run it against a large body of community projects to check for breaking changes and edge cases. You can see some of the work they do in this video: https://www.youtube.com/watch?v=zvjOT7NHl4Q