Weelllllllll. The first Guild Wars games were quite financially successful.
The problem is your concept of :success: is pretty vague and something that exists mostly in your imagination. How deep down this rabbit hole of qualifiers do you want to go?
What arbitrary amount of money is the tipping point for this "threshold"?
From what we know factually, this model of design is both successful and sustainable for years. So much so as to develop a sequel over 5 years with exponential costs and development over the initial series.
Perhaps Arena.net will make billions and Colin will buy a swimming pool of diamonds. Izzy might show up to work in a sabertooth tiger fur coat.
Or they might be successful enough to continue development over several years and expansions at a pace on par with what was seen in GW1.
One of those scenarios is high fantasy.