Why does null exist in .NET?

We've got Tony Hoare, an early pioneer that worked on Algol to thank for that. He rather regrets it:

I call it my billion-dollar mistake. It was the invention of the null reference in 1965. At that time, I was designing the first comprehensive type system for references in an object oriented language (ALGOL W). My goal was to ensure that all use of references should be absolutely safe, with checking performed automatically by the compiler. But I couldn't resist the temptation to put in a null reference, simply because it was so easy to implement. This has led to innumerable errors, vulnerabilities, and system crashes, which have probably caused a billion dollars of pain and damage in the last forty years.

A billion is a low-ball number, I think.

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UPDATE: C# version 8 and .NETCore have a decent solution for this problem, check out non-nullable reference types.

As appealing as a world without null is, it does present a lot of difficulty for many existing patterns and constructs. For example consider the following constructs which would need major changes if null did not exist

1. Creating an array of reference types ala: new object[42]. In the existing CLR world the arrays would be filled with null which is illegal. Array semantics would need to change quite a bit here
2. It makes default(T) useful only when T is a value type. Using it on reference types or unconstrained generics wouldn't be allowed
3. Fields in a struct which are a reference type need to be disallowed. A value type can be 0-initialized today in the CLR which conveniently fills fields of reference types with null. That wouldn't be possible in a non-null world hence fields whos type are reference types in struct's would need to be disallowed

None of the above problems are unsolvable but they do result in changes that really challenge how developers tend to think about coding. Personally I wish C# and .Net was designed with the elimination of null but unfortunately it wasn't and I imagine problems like the above had a bit to do with it.