"Especially" doesn't mean "exclusively".
As to the other points, since we're all about defining here; that definition isn't 100% clear. It opens up a bunch of other questions such as
1) how much excellence of form or expression?
2) as defined by whom? as measured how?
3) what qualifies as an idea of permanent or universal interest?
4) How permanent? How universal?
etc. Even by that definition, I could gladly argue that any book from "Where's Waldo" to "Finnegan's Wake" matches even your definition.
Also, age cannot come into it. That would imply that the exact same book is not literature up until it reaches a certain age, at which it magically becomes literature overnight.
As to WHY we would need an exclusive definition of literature... I still say it's not nearly as important as being able to define WHY certain works hold more quality than others. Just like I'm not going to use a narrower definition of "food" or "music" to exclude Big Macs and Bon Jovi - it's much more interesting to try and explain why a good meal or a great record has so much more to offer. What would be most useful to learn in school; a well-defined list of works that are considered "literature", or a way to learn to recognize quality on your own?