Graal is not a HotSpot replacement. It's a JIT for HotSpot or an AOT compiler for SubstrateVM which is a separate JVM altogether. If Graal matures and proves itself, it will become HotSpot's JIT. And Substrate may or may not become a product regardless.
And project Sumatra -- while cool -- was never a big influence over OpenJDK's plans. Being able to run streams on GPUs is absolutely awesome, but not the number one priority for the majority of Java users. My point is that Sumatra wouldn't have played a significant role in the decision of when to make Graal HotSpot's default JIT.
BTW, you don't even need Graal to be the default JIT in order to support Sumatra, anyway. Graal as a plugin (JEP 243) is good enough for that.
It's not the whole JVM -- just the JIT. What difference does it make what language the JIT is written in? It is my understanding that if Graal proves itself, it will replace C2 (if not C1 as well).
I don't know, lets see how it turns out.