I mean the fact that they've been around since 2.0 but you saw it used heavily in new codebases says a lot doesn't it?

You misunderstood - I saw them in codebases that were new back when .NET 2.0 was new.

To be more precise, it was when .NET 3.5 was new (2008). That brought a lot of visibility to Nullable<T> because it was featured prominently in LINQ. But ?. didn't come until C# 6 (2014). I was wrong about ??, though - that one was added along with Nullable itself.