In fact, C# deals excellently with legacy code. While, unlike the JVM, .NET is not binary backward compatible (allowing some excellent language improvements that Java still has not managed to pull off), it is nearly (but not entirely) source code backward compatible. Sure, this allows to old code using old constructs to stay alive, but it works. That alone is pretty unique, especially if you see that nearly any other language either does not manage to move its community to the next version with breaking changes (Python 3, PHP 6) or has tons of compatibility issues every time something changes (Ruby, Scala), or simply is at a standstill (Java).
Having to deal with a DictionaryBase child class which was made before generics were added is way less of a deal than not being able to upgrade your software to the newest version of the language at all. Which is what would've happened if this was done like Scala. Or, if it was done like Java, we'd still be hacking DictionaryBase child classes.
I'd maintain that no language/platform deals with legacy code better than C#.