Think of all the millions of man hours that have gone into SQL Server, Oracle, PostGres, MySQL, Teradata, Neteeza etc. Imagine waking up one morning and thinking that you could do better!
Either you have a very specific use case which doesn't require 99.9% of what one of the main protagonists need or you are Michael Stonebraker.
I've a theory that a DB has to get beyond version 8 before it can be said to be mature. SQL Server 9.0 was SQL2005. That was a huge step change. I've seen blog posts saying that Oracle wasn't that great before V8.0.