It might have vulnerabilities that a motivated attacker could discover by reading the source code. It might contain copied code that Microsoft doesn't have the right to distribute.

Not saying these are good reasons, but in a company whose main products are closed-source software, I'd expect some PM to make these objections.

> It might have vulnerabilities that a motivated attacker could discover by reading the source code.

I'm sure the flipside of that, that lots of people looking at the code will uncover the vulnerability and fix it, more than compensate for that.

> It might contain copied code that Microsoft doesn't have the right to distribute.

This is why opensourcing an existing codebase can be complicated, but, if you start something managing the licenses of included code, then it's not a huge issue.

> in a company whose main products are closed-source software, I'd expect some PM to make these objections.

I'm sure they'll never opensource Exchange, SharePoint, or SQL Server.