We'll know in a decade. It's too soon to tell whether Microsoft has something up it's sleeve. (The open source stuff is always a version behind Microsoft's latest, maybe?)

I've watched Microsoft since the 1980s. One year is way too soon for me to believe that they've changed their stripes. In particular, it's way too soon for me to want to tie myself to their platform.

embrace, extend, extinguish

Why would you trust a company like microsoft, when there are tons of true free/opensource software available?

Because their tech stack is pretty good from developer experience point of view. Good in the sense developer speed is good, as in speed in developing new features, understanding the code and debugging. There are not "tons" of good industrial strength free/opensource alternatives to .net stack. There are, but I've not seen one that would be of better quality. In ways Visual Studio is the gold standard of developer experience (unless one goes shell and makefiles all the way but that's... a different demographic I suppose).

At moment they only open-sourced core that only useful for running backends on non-Windows platforms which is useless on Mac as nobody run servers on it.

Once their open source .Net would get most of Windows version features there may be some use for it with same drawbacks like you have with Java for example.

.NET Core is available on Linux, FreeBSD, and Mac, so you can use it for servers on Linux.[1] A lot of developers (such as myself) use Macs for their development machines, so it's very useful to have the same framework/runtime for both Mac and Linux.

[1] https://github.com/dotnet/coreclr#get-net-core