The biggest design mistake in Java was the omission of stack allocated value types, and the same for C# was the distinction between value and object types. If something can be done on the stack it should be done on the stack - the biggest lie we were told was that "short lived objects are cheap".
Or maybe I'm in a total echo chamber. Working with a client today, they were astonished to find out that:
var x = new List() { 1 , 2 };
var y = x;
x.Clear(); // This affects y.
So who knows, maybe MS's overwhelming feedback is from people that fundamentally don't understand what's going on, and they're the ones with money?
To be fair, I've seen successful C projects with lines like: buffer = malloc(....); // gets memory
(n.b.: when I write C#, it's for game development, and I am constantly thinking about structs versus classes and I'm very careful about them, so my perspective is probably not the same as your average C# developer.)