SQL Server, like Oracle, is technically fantastic. They were both ahead of Postgres for a long time and many people would argue they still are.
The reason people don't use them more often is that they're not free or even inexpensive.
I mean they should be, they backed by huge corporation
plus they can take notes from Open source DB like postgress and improve their system better
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.