I think it depends on who you are. In 2007, a lot of people distrusted Microsoft and with good reason. They'd spent a lot of time extinguishing competitors. .NET Core was initially released in 2016, but Microsoft wasn't saying that .NET Core was their plan for the future. It seemed like an experiment they'd likely kill off or that it might just be a small subset of .NET.

I think saying that Microsoft started open sourcing .NET in 2007 feels a bit disingenuous. Plus, wasn't it source-available under the Microsoft Reference License? Regardless, .NET was still tied to Windows unless you wanted to use Mono (which was slow and had an uncertain future).

If you were someone who developed on Mac or Linux and deployed to Linux, you couldn't choose .NET in 2007 or 2014. Even in 2016, are you going to choose a brand-new ASP.NET Core? Microsoft announced .NET Core 1.0 at the Red Hat Summit in 2016 which isn't exactly an endorsement that the company thought it was the future of .NET. It seemed like Microsoft was trying to open up just enough to hook a company on .NET Core and then tell them "well actually, you should really upgrade to the real .NET Framework on Windows" when they ran into problems. That's not what happened, but Microsoft certainly hadn't committed to .NET Core in 2016. Their messaging was "well, Red Hat will offer support for this thing we made."

If I didn't have a Windows PC in 2015, I couldn't do .NET development (Mono aside). If I wanted to deploy to Linux in 2015, I couldn't do .NET development (Mono aside). The open source part matters to lots of people because .NET simply wasn't a choice for those who didn't want to be beholden to Windows and licensing until recently. People definitely ignored .NET because it simply wasn't an option for them.