I'm a fan of the original Portal - you don't have to be "old school" to have enjoyed the SP.
I'm sorry too - that the fact that you're ignoring a good third of the content included in the price of a game (and clearly advertised as included) doesn't make it a short game.
I think you're comparing apples with oranges TBH.
Portal is a primarily SP game and that is probably what most people will buy it for.