Erasure makes it easy for Java, Kotlin, and Clojure share code and data structures without costly runtime conversions. Languages like Scala and F# have had trouble implementing features on the CLR because of reification, and take a look at what Python and Clojure on the CLR have to go through for interop.

I know some people think that say they like reified generics in C#, but those are mostly people who aren't aware of what cost to the entire ecosystem they're paying in exchange to what is a minor convenience in C#.

BTW, reification of reference-type generics is not to be confused with specializing collections for value types ("arrays-of-struct") an extremely important feature that CLR indeed has, and Java is now working on getting.

"Mistake" is a strong word, and I would disagree with it. But when building extensible libraries and systems I do often find myself wanting to pass around a List<Something<?>>, which isn't doable without writing a second generic interface or the like. On the flip side, in Java (or, these days, Kotlin), a type-erased generic can be mediated more easily because passing a Class is easier than adding a second interface.

Part of it is that a lot of that stuff is gamedev-related, for me. It's not the biggest thing in the world, but for a lot of the stuff I find myself writing in C#, I find myself wishing for type erasure to make throwing data around a little easier. On the other hand, though, when writing web stuff on the JVM--I absolutely will not waste my time doing this in .NET, ASP.NET Core is not very good and EF Core is awful--I often wish for type reification, so it's just a horses-for-courses thing.