Could they do much more of it without going over the
  top? It's not that clear.

Star Wars manages to produce prodigious amounts of merch and people eat it up. 188 different Lego sets [2]. 70+ trading cards [3]. 150+ video games [4]. Innumerable tee shirts [1]. They'll even put their brand on bags of fruit.

You can extract a lot of money from fans - especially if your franchise has inter-generational appeal.

[1] https://www.google.co.uk/search?q=star+wars+shirt&tbm=isch [2] https://en.wikipedia.org/wiki/List_of_Lego_Star_Wars_sets [3] https://en.wikipedia.org/wiki/Star_Wars_trading_card [4] https://en.wikipedia.org/wiki/List_of_Star_Wars_video_games