Yeah, the library problem has some (ugly) solutions in some languages, like Go allowing multiple versions as you mention, or doing JAR shading in Java. But I do

Yeah, the library problem has some (ugly) solutions in some languages, like Go allowing multiple versions as you mention, or doing JAR shading in Java. But I don't think there's a (mainstream) language that solves the problem of partially upgrading the underlying runtime. If you've got a Java monolith, it either all runs on Java 7 or Java 8, can't really do in-between.