You’re right that the whole experience is about unique decks. If the starter set were £15 with no unique decks then it would lose a lot of its appeal!
On the other hand, I’m happy with the purchase even though I could have bought 4 unique decks for only slightly more than the starter set. I enjoyed playing with the starter decks and I’m looking forward to opening the unique decks. If I hadn’t already played with the starter sets, looking through the first unique decks wouldn’t be as exciting as we’d have nothing to compare them to. It also helps the game remain accessible when it comes with everything you need (apart from a full rule book, which is still disappointing)
Since I don’t play any card games I think the novelty of the starter set makes it a decent value purchase and easier for me to enjoy. I could see that more experienced card game players (Magic, Netrunner etc) might not feel the need to get the set if they are familiar with the genre!