Both ecosystems complement themselves, some stuff Java ecosystem does much better than .NET, like multiple implementations, with various kinds of GC and JIT implementations, wider support of hardware deployments, tooling like Graal, industry standards, IDE implementations, a mobile OS,...
Other things the .NET ecosystem does better, support for value types, low level programming, SIMD, desktop development, game engines.
To this day they keep copying features from each other, and if either C# or Java isn't one's taste, there are still several other options on each platform.
Hence why I am confortably at home using them both, complemented by JS/TS for FE, and C++ for fiddling with their runtimes, or plugging into native libraries.
Most of C# 1.0 for starters, given that it was born from the lawsuit and J++ extensions, J/Direct, COM interop, events and Windows Foundation Classes.
More recently, default interface methods, apparently the ongoing discriminate union design seems to now be settling on similar Scala/Java approach, I still have to spend some time delving into it.
Not delving now on the CLR, versus the various JVM implementations, regarding GC, JIT, PGO, tiered compilation, escape analysis,...
