As with all carriers that offer US (and Canadian) numbers, they are required by law to let you port your number away. So you can always bring your number elsewhere if they decide to close up shop.

And if that happens, likely someone will setup another instance and keep the same service running. That's another great advantage of JMP over Google Voice: all the code is free and open source, so anyone is free to run their own instance if they like. In fact, you could run your own instance right from the start and then be in complete control of whether JMP keeps working for you.