I think there are two major reasons so many of these MMORPGs change themselves over to the "free to play" model. The first is due to competition and the second is due to initial investment.
Many of these games are very similar to one another, to the point of being somewhat indistinguishable to the cursory eye. Additionally, they also all have relatively little content, with most of it being extremely easy (there is enough to keep a hardcore player engaged for a few months before most everything has been earned, I mean). This caustic combination of similarity to others and little depth creates a situation wherein people unsubscribe as soon as the next MMORPG rears its head -- there's no motivation for them to remain, and the upcoming product makes them feel right at home.
The second problem is related to the first and involves a company's investment in the game's creation. Games cost more now to produce than they ever have before, due to all sorts of reasons that I'm sure everyone knows. This means that, in order to survive, an MMORPG needs to eventually earn money, which becomes a hard thing to do. Add to this the earlier problem of "too little content," and we see how these games have trouble generating significant revenue, especially over time.
In sum: so long as games continue to copy one another so heavily, and continue to provide primarily casual-oriented experiences, they will all be chasing the same, fickle user-base: one which will never be likely to subscribe for more than half a dozen months. Like EVE, FFXI choose to set its sights on a certain market; they filled a niche and remain extremely profitable to this day. Time will tell whether any post-WoW developer manages to tear his or her eyes away from the inimitable 10-million subscriber goal -- until they do, they'll only be preparing for a situation that will never come, and continue to fall flat into the "free to play" ditch.