PMML allows you to use models generated by a variety of tools and systems, including external vendors. While you are right that making it core does create some constraints, it also allows them to easily replace portions of the model building pipeline with relative ease.

Yes, PMML isn't perfect (being kind), but it continues to be extended and is the one shared lingua franca we have across model creation systems, short of (sigh) SAS code and "recode the model in generic C", both of which I see too often.

I suspect in the future we'll see "standard" architecture with pipelines with multiple parallel feeds and runtime engines into ensembles, each of which allows various model types in "native" format (sklearn and other pythonics, R, java, etc.) which would be interesting, instead of having to cram all into PMML. Just a thought.