It's all about finding the right tool for the job. For most, this audience especially, open source relational databases are the way to go. For a lot of big business and government jobs, Oracle and Microsoft are the way to go and having your preferred ops environment makes a big difference. At my first job we had to maintain an Oracle DB connection to pull data directly from the City of Los Angeles. At the end of the day we were out of business without that so we didn't really debate the merits of using it. That job happened to be a Windows shop anyway (most of the company were CAD artists using Architecture software) so it didn't phase the IT ops team.

On the other hand, at my current job I've been in charge of some search tech that's Windows only and being the lone Windows user has been a bit painful. We liked the results it gave us but recently developed our own algorithms on top of ElasticSearch so we didn't have to deal with Windows anymore. The Dev work to switch was more than the cost saved on the license for the old software, so this wasn't a case of open source saving is money on the solution implementation. We really just didn't want to focus on Windows Dev ops. I think this is the concern the SQL Server team had and why they're supporting Linux, especially in Microsoft's increasingly Azure focused world.