I don't think languages, large organisations, and backwards compatibility work in the way that you imagine.

Java 8 is still probably a minimum of a year or 2 off. After that, it will go through another year or two of testing before any big corporation will dare even bother touching it. After that, it might start being used for new apps, but it will be another 2-3 years before it has widespread adoption.

On the flipside, what they've done isn't exactly groundbreaking, and could be used with Java 6.

They don't want lambda just because they want lambda. They want function collections with filters and transformations, and you can't have that without lambda. When Java 8 lambda syntax arrives, it will be compatible with this and Guava.

They also likely have no interest in using JRuby because it isn't statically type safe and therefore adds another category of things that can go wrong at run time that is completely unnecessary to test for when using Java with Generics.