Let's say that among developers there was a history of how-to-implement gameplay traditions (like how to implement third person player movement, "gameplay ability systems", etc.) in programming languages besides C++. Like C# & Java, the memory managed friendly ones with good tooling. And let's say you're forbidden from reinventing C++ inside C# or Java, like Unity's Burst does (so called HPC#). But you can "do ECS," there are C# and Java ECS frameworks, that even use those languages respective arena allocation techniques well. You just aren't allowed to reinvent C++, but you can use a high performance middleware that does, like Netty.

Would you choose to author an MMO backend in one of those friendlier ecosystems?

Do you think there's value in having access to other Java applications, to embed as libraries of your grander "in memory" ideas?