I think that this is a technical change.  I understand that you are inferring things from the fact that the text permits RST_STREAM to cancel all pushed streams, but this expands that further.

From a layering perspective, tying the lifecycle of streams together in this fashion is problematic.  It means that it is no longer possible to process FINAL flags or RST_STREAM without having some sort of allowance for these side-effects.  That means the overhead of maintaining an observer list for closed streams, or other special handling of these two cases.

It's a technical position, but I want to see this fate sharing go away.