>> RDBMS has matured? I feel like I'm seeing a new database pop up every quarter.

RDBMS's have been in use for more than 50 years:

https://en.wikipedia.org/wiki/Relational_database

There are hundreds of implementations some of which are decades old:

https://en.wikipedia.org/wiki/List_of_relational_database_ma...

Oracle and several other companies' (some of which are now defunct or were acquired by other companies) primary products were RDBMS software.

New implementations do not indicate lack of maturity but perhaps the wide availability of tools and libraries to build RBDMS's and their utility.

Perhaps you are referring to database systems in general and not relational databases?

Right my bad, I was thinking of storage engines in general. And of course maturity doesn't mean lack of innovation - which is still happening in the relational space.