There is nothing 'just' about moving a non-trivial database from one rdbms to another. First there is no doubt a huge amount of incompatibility between the dialect of SQL the two speak. Then there's all the platform specific features that the other database is either lacking or implement completely differently. On top of that, once you get everything up and running, you'll notice that the performance characteristics between the two are very different which means you're going to have to redo much of your optimization work to get back to even close to the level of performance you had before the switch.
you would think MS would cut them a super deal on licensing, being that S.O. is such a major site utilizing and promoting their platform, (and its demographics match who they'd want to promote to). If S.O. goes down or has problems due to a SQL server issue like this, (or them not affording another license) that looks pretty bad.
I'm inclined to agree. If you have to spend any time at all figuring out how to fit the cost of a SQL Server license into your business plan, that's the entrepreneurial equivalent of a code smell.