Just providing these features for new libraries is not enough; if widely used libraries can't compatibly evolve to take advantage of this new world, they are effectively consigned to a slow death. This might be OK for some classes -- deprecating OldX and replacing it with NewX -- but only when uses of the OldX types aren't strewn through lots of other libraries. That rules out starting fresh with Collections, Streams, JSR-310, and many other abstractions, without rewriting the whole JDK (and many third party libraries). So instead, we have to provide a compatible path for such libraries (and their clients) to modernize.
So, to summarize: Valhalla may be motivated by performance considerations, but a better way to view it as enhancing abstraction, encapsulation, safety, expressiveness, and maintainability -- without giving up performance.