Why would anybody care?
Color me cynical, but I fail to understand why anybody would care about this in the slightest. There are like a billion calculator programs out there, and half of them are already open source and cross platform. It's not like a calculator is complicated. I wrote one in VB3 when I was about 12.
It seems to me that this is the latest in a long line of empty gestures, where MS open sources something nobody asked for or cares about and makes a big song and dance about how they're soooooo into FOSS these days and soooooo not like the old, evil MS. Meanwhile you look at their calculator and it slurps your data and phones home, and they're yet to open source a single thing of any consequence (you might think that .net core is important, but we already had a more full-featured FOSS .net implementation before that - mono, making it redundant. Maybe you think visual studio code is important, but we already had a million full-featured IDEs. VS code running on Linux is merely an ad for windows - if you like this and you want all the features enabled, install Win10 and pay $whatever). MSSQL is arguably something that somebody might care about, but there were already better FOSS database engines out there.
I've been saying it for years, and I'll say it again: Let me know when they open source DirectX, then I'll give a crap. But that will never happen because 10 minutes after they do it'll be compiling on Linux and 10 minutes after that porting any game to Linux will be truly trivial.
(I'd *love* to be proven wrong about this, BTW, because it would mean many more games would get ported)
If MS want to prove how into open source they are, they'll need to actually open source something useful and important. Then maybe I'll believe them. But as far as I can see everything they've done so far is just spin. The sad thing about it is how many people are buying into it.
As a bonus, they've now offloaded the whole writing code thing to a team of volunteers.