I hesitantly agree with your sentiment for the fact that I can see myself either keeping just the original skin or buying a skin because I love the way it looks and a Visual Update may remove what I loved about the champ. So I can see value in being able to keep skins/champs looking the way they did when you bought them, or having the option to do so.
On the other side, Visual Updates often fix mapping/collision/hit box/thematic bugs and you would be keeping those bugs in by keeping old looks.
So I am on the fence, but I know a few champs I have that I would NOT want to see changed, and I can therefore see the value in being granted access to the old look/skin. Also, do you mean access to the old Splash Art?