"The absence of a monthly subscription seems to empower ArenaNet to concentrate on making the game as enjoyable as possible for however long each player chooses to invest."
This is the advantage of traditional pay-once games -- the developer's primary objective is to create a great experience for the player so that they'll pay for the game and tell their friends.
Whereas the primary objective of subscription and free-to-play games is retaining players for long periods of time so that they keep paying more and more small fees. Such games often prey upon frail human psychology and, while addictive, are often less fun, less interesting, and less enlightening experiences.
I'm not saying that the two objectives are mutually incompatible, but it *is* very challenging to create subscription and f2p games with both longevity and per-minute gameplay that is as fun, interesting, and enlightening as pay-once games.
We're still failing more than we're succeeding in the f2p space, but games like League of Legends, World of Tanks, and Team Fortress 2 show that it can definitely be done.