I agree with you that if they stop adding new content then subscription fees are not justified.
Dropping the monthly fee won't keep veterans on the game who have run out of things to do. It would probably lure some newer players into the game though. So going F2P might stretch out the life span of Final Fantasy XI if it is their intent to pull the plug soon anyway.
These are answers quite apart from should they . The old arguments against f2p models are valid. There is a reason f2p games require less staff, development, and investment. They are basically the Wal Marts of the industry - sacrificing quality for lower upkeep costs and higher profits.
My understanding is that SE has said only that they would stop creating new content for the Xbox and Play Station 2. Yet everyone on these forums seem confident this game is basically dying in November. So I'm not sure what to believe or what is happening.
I'm just going to play this game until there is nothing left for me to do. When that happens I'll find something else to play. I have decided there is no point worrying about the future.