I’d recommend spending some more time with .NET Core and comparing its performance characteristics (as of .NET Core 3.1). I think you’ll find it compares quite favorably, with recent advances like Span making a big difference. While it’s accurate to say the CLR has been behind on the desktop for a long while, the rekindled effort around advancing the runtime in .NET Core has paid off in spades and has only really just begun. It sounds like your impression is rooted in the desktop CLR, which is effectively frozen for compatibility reasons.
I'm sure it has, but in the meantime Java is making great strides as well on multiple important fronts -- low-latency GCs, a next-gen JIT, AOT compilation, native ffi, low-overhead in-production profiling and lightweight concurrency, and the main problem with Microsoft's technologies remains that they break compatibility every five-six years or so. Now it's .NET Core, and it wouldn't be a bad bet to guess that in five years it will be something else. We're at a point where it's so hard to make changes with a big bottom-line impact (arguably there weren't too many such advances in the past twenty years) that very few actually justify breaking compatibility, and companies know that. So while smaller software can afford switching from one technology to the other, big "important" stuff requires a compatibility lifespan of at least ten if not fifteen years, something that Microsoft has never been good at. It's possible that with their focus shifting more to the backend with their cloud offering that would change, but it appears that their cloud strategy is to support all software platforms rather than focus on their own. That's why we see Microsoft now hiring engineers, as well as getting some CLR engineers, to contribute to Oracle's OpenJDK [1].
Well, you're certainly entitled to that opinion :)
As I said, I encourage you to explore .NET Core and its recent advances, especially now that the big runtime improvements made in recent years have proliferated throughout the standard library and new language features. While it's fair (and correct) to say that the Java runtime is ahead due to many years of focused improvements, I think you'll find the difference quite a bit smaller than it was back when .NET was focused mainly on Windows desktop software.
I believe you re .NET Core, but I think that Microsoft shifting their focus to a backward-incompatible development platform (not Windows, on which they have a very good compatibility record) every five years or so is more than just opinion. I think that their record on that front speaks for itself. If you were a developer who always uses the current flagship MS development platform to develop a piece of software that you first wrote in 1999, by now you'd be on your fourth significant rewrite. TBF, Java also made one such misstep, with JavaFX Script in 2009, but that was corrected and reverted.
That may be your impression. I'm only speaking from experience here, where we have done things like investigate issues with the first public release of a compiler for a week to ensure behavior is still correct when used in a newer environment.
But it's important to distinguish where we are now - .NET Core being explicitly cross-platform, vendor neutral, and OSS - from where .NET was. .NET Core is not tied to a specific product or initiative (e.g., Silverlight) that could go down due to other market forces and take that flavor of .NET with it. This is the position that Java has been in for a long time. Perhaps you or others you've known have been burned by buying into a Windows Mobile Flavor of the Month only to see it canned a few years later, so it's not unreasonable to think that the same could happen here. But at the same time, .NET Core has been going for 5 years now, and over that span of time it's only become more compatible with existing APIs and runtime environments, not less.
