To answer your questions about deleting accounts and informations :
Deleting our online posts and accounts is hardly the same thing as deleting the memory of yourself from other people's mind : It's actually deleting the possibility for other people in the future, people who weren't there at the moment you initially wrote those things, who didn't experience the original context, to read a full log of whatever you wrote at that moment in time.
There's this notion now that what we write online is just "us" and deleting it is deleting "ourselves", but it's not. Our writings are statements, thoughts, they could even be compared to exploded bits of diaries or books. And I don't think we should blame anyone for not wanting to see some statements they now disagree with or simply feel are not relevant anymore available to everyone to read forever. Those who read it when it was online will still remember them, we won't erase it from their memories and if that knowledge was useful to people at some point, deleting it won't suppress everything it helped building.
It will just allow the person who deletes it to give a better idea of who he/she actually is now, what kind of message he/she wants to present to the world, and how.
To come back to your "Eternal Sunshine" analogy, I think it's not the correct drama you're quoting. The "real life" analogy of a world in which we couldn't erase stuff we said online would actually be the Black Mirror episode (Season 1 Episode 3 wikipedia tells me) where everything we do is always recorded in a harddrive in everyone's brain and we can always go back and check the log and dissect everything we've seen in our life, so basically, no one can ever get away with anything they've done, and I think nobody wants that either.
Here's my take on "why anyone would want to delete his account permanently from somewhere". I could go on and on because there are many other reasons, but I think this one is enough on its own to justify it. It's a subject that matters a lot to me so I felt a bit strongly about it, but I hope this long message didn't sound mean or anything
Apart from that, I've never been to MW, so I don't have anything to say on that.