Steve, I think you "hit the nail on the head" when you said that SQL Server includes a lot of features beyond the database engine. If you're coding to ANSI standards, then moving the actual database code shouldn't be too much of an issue. However, it's all the goodies, that make the whole SQL Server ecosystem, that make SQL Server such a compelling platform to work on.
I've looked at a few other database engines. And they all seemed reasonable. But, then you look at the extra tools, and it's like, "Um... I'll stick to SQL Server." I can do in a few clicks, what will take a good deal of programming to do in some other product. Or, worse yet, can't be done in other products. Yes, the ecosystem is a very compelling argument for staying with SQL Server.
MKE Data Guy