> Array.FindAll, Array.Convert, etc. all existed in C# beforehand. Though maybe this is what you meant in the next sentence.
Yes, it's what I had in mind. I have to admit that I completely forgot about ConvertAll (and so assumed there was no map).
I think the biggest reason why those weren't all that commonly used in practice, is because in .NET you often deal with opaque collection types (like ICollection, or ReadOnlyCollection, or even custom-made collections pre-generics) that are usually exposed on properties of objects. Since the concrete type is not known, you can't do List.ConvertAll etc.
This, by the way, is another point in the favor of lazy implementations - they don't care about input type, because the output type is always "lazy sequence". Of course, you can have an eager map similarly not care about input, but then what should be the type of its output collection by default? No matter what type you choose, someone will complain that they wanted someone else. Given Python's preference for explicitness, such design would warrant several functions like map_to_list, map_to_tuple, map_to_set etc. But, of course, if you have a lazy map, you might as well just write list(map(...)) etc.
> You have to say "using System.Linq;" at the top if you want to use the new syntax. That's like saying "from itertools import " and then using imap, which you could've always done.
It's a bit different, though. When you import itertools, it brings all those functions into your global namespace. But when you import System.Linq, it only brings one static class into your global namespace; the actual functions are extension methods that only show up on the types to which they are applicable. So the resulting namespace pollution is far less in C#.
There's also the issue of import being generally frowned upon in idiomatic Python, largely because the way conflict resolution works there (silent override). In C#, if you happen to have clashing identifiers from usings, it'll prevent you from using them unqualified, so there's no good reason to avoid it.