It would be tempting, but there are too many 'what if' scenarios. $500 is a lot of money for some people. I'm the kind of person who pays for a six-month car registration that is $8 more than half the cost of a 12-month. I'd rather spend that extra $16 a year and go in smaller increments than dish out more money right now for more time on something. Because no one can predict the future.
In hindsight, it would have been great to have paid $500 in 2005 and never had to pay a subscription fee again. But looking to the future, I honestly could not say that I will play consistently for three more years.
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I feel that way now. The biggest reason I don't play another MMO is that I have invested too much into WoW over the last nine years to say goodbye completely. :/