After this merge we will have JVM implemented mostly in Java (aka "self hosted"). Currently about 1% of the JVM runtime is implemented in C and this will be replaced with instrumented Java code (including garbage collectors). This means that only one runtime will be maintained in the future and Java developers will do it (and not C/C++ developers which are harder to get). It will be much easier to implement and test new memory management mechanisms than it is today.
If they also decide to merge support for other languages (including WASM and LLVM) we will be able to run everything that compiles to WASM and to LLVM on the same runtime - in the same JVM. So we will get another "Docker Without Containers"...
EDIT: It seems that I am wrong with the above 1% statement. I meant that about 1% of the Java standard library is written in C/C++. Regarding JVM: I guess that the whole HotSpot VM will be replaced with GraalVM in the future and when this happens, majority of the JVM code will be in Java.
The JIT is probably half of the VM and is written in C++.
Github [1] says (for the whole JDK, not just the VM): Java 75.1% C++ 13.4% C 7.4% Assembly 2.3% Objective-C 0.4%