None of these are good technologies if you want to stay up-to-date. You could also write your site in IE6 compatible HTML which was quite mature but that seems a really bad idea.
That's not a reasonable comparison at all. Winforms is maintained. IE6 is explicitly deprecated.
I would argue that Winforms is now a mature technology and requires little upkeep, but it is still a part of the stack.
WPF development has slowed significantly, but still proceeds.