The rise of frontend developers over the last 5 years learned everything must be new.
That a math library of all things could be complete is several orders of thinking beyond their ability. I'm sure the gut reaction is to downvote this for the embarrassing criticism, but in all seriousness, this is the right answer.
Sure code can be “feature complete” but the reality is the rest of the world changes, so there will be more and more friction for your users over time. For example someone in the issue mentions they need to use mainline to use eigen with cuda now.
Mathematics is a priori. It's beyond the world changing. You might be surprised to learn we still use Euclid's geometry despite it being thousands of years old.
What you're actually saying is you expect open source maintainers to add arbitrary functionality for free.
I don’t think I’m saying that at all. There are plenty of little libraries out there written in C89 in 1994 that still work perfectly well today. But they don’t claim to use the latest compiler or hardware features to make the compiled binary fast, nor do they come with expectations about how easy or hard it is to integrate. The code simply exists and has not been touched in 30 years. Use at your own peril.
If you have a math library that is relying on hardware and compilers to make it fast you should acknowledge that the software and hardware ecosystem in which you exist is constantly changing even if the math is not.
Randomized linear algebra and under-solving (mixed precision or fp32 instead of fp64) seem to be taking off more than in the past, mostly on gpu though (use of tensor cores, expensive fp64, memory bandwidth limits).
And I wish Eigen had a larger spectrum of 'solvers' you can chose from, depending on what you want. But in general I agree with you, except there's always a cycle to eke out somewhere, right?