The MS SQL Server engine is an improvement over the years of a branch that was always more technically advanced than Postgres. I don't think there is anything to steal from it.
You have it backwards. FOSS projects are still currently learning from SQL Server and Oracle. That may change in the next 5-10 years as Postgres has already become the lingua franca of the DB world.
plus they can take notes from Open source DB like postgress and improve their system better