You know, I've had a fair number of services that I'm paying for just vanish out from under me, too (e.g., eWorld, iTools, Grove.io, the original Siri). Sometimes the wind-down is longer, sometime it isn't. And, conversely, some things I don't pay for have been around for a very, very long time (Freenode, Gmail, my old Fidonet BBS, etc.). Maybe paid sticks around longer on average, but I think paid-v.-free is a bad litmus test.

I think there's a much better one: how easy it is to take your data and go home. With Trello, the answer is "exceedingly easy." On any board, just go to "Options" -> "Share, Print, Export..." and dump the entire board to JSON. From there, it should be easy to move your data onto Remember the Milk, Pinterest, org-mode, or whatever's appropriate for you. You can even automate grabbing that JSON on a daily basis via the API, if you're so inclined.

So while I understand your argument, your actual risk here is very, very low.

As another Creeker pointed out elsewhere in this thread, the Trello team has some stuff coming out in the future that you may find appealing, if you would feel better contractually paying for stuff. But in terms of risk, I think that it can be very low, today, whether you pay or not.