I think it makes sense to keep up with the latest version of the programming language you use, and keep updating the codebase.
The important part is not introducing an entirely new language that no one knows, or changing around the entire architecture without really being able to understand the long term consequences.
Also, after moving to C#, I don't know why anyone uses plain old Java anymore. C# runs on Linux now. It's not the 90s anymore. Oracle is the dinosaur and MS now is kind to its developers. Plus I'm hoping that client-side C# will take off with WebAssembly.
As a previous fan of C# and a long time Java sufferer, I'm pretty sure it's too little, too late for C#. Sadly, years of being locked to Windows has stunted the C# ecosystem and given Java enough time to get less horrible.
Java 8+ is okay. Lombok helps. There's other great, mature libraries around. There's a choice of IDEs. There's JREs to choose from - there's even JVMs to choose from. Gradle makes build files not suck and makes it easy to add static checking and linting to the build process. AFAIK, C# doesn't have auto-formatting options outside the IDE.
As much as I used to have your view, Java is alive, works fine, and makes money. Or Minecraft mods.
Because there are plenty of platforms where Java runs and C# does not, the world is not constrained to Linux and Windows.
Copiers, phone switches, IoT gateways, SIM cards, military control units, drones, Intel ME and a couple of phones.
And all of them have libraries that work across all JVM implementations, regardless of the OS, not the hit-and-miss that still happens between Framework and Core, without a good story how to go cross platform [1].
As for Oracle, they rescued Java while no one else cared [0], kept Maxime alive transforming it into Graal, made several improvements to the language, while allowing jars from 20 years ago to still run unmodified in modern generations of the runtime.
I use both platforms since their initial releases. Each of them has plus and minus.
[0] - Only IBM did an initial offer, that they withdrew afterwards.
[1] - Naturally there is Xamarin, but it isn't without its issues and doesn't run everywhere where Swing, SWT or OpenFX run.
The important part is not introducing an entirely new language that no one knows, or changing around the entire architecture without really being able to understand the long term consequences.
Also, after moving to C#, I don't know why anyone uses plain old Java anymore. C# runs on Linux now. It's not the 90s anymore. Oracle is the dinosaur and MS now is kind to its developers. Plus I'm hoping that client-side C# will take off with WebAssembly.