It is really hard to answer that clearly, personally for me if XF hadn't come along I would have been done with the forum scene completely so no I have never considered moving.
If you go to another forum software they will pretty much only show you/make available to you methods to import to their software.
This is for no other reason than anyone with any interest in specific software (biz and clients alike) has an interest in people being there and it actually makes little sense to build a way to move out from it and it is no different than any other forum software's community would practice (sans private developers).
You can literally import/export from any forum software to any forum software supposing that it has a db and some kind of organization of data. The only thing is someone has to code a script capable of doing so and every permutation of source -> exported forum software potentially needs it's own methodology.
To ask if someone that I don't know who is representing a forum software that is unnamed has the ability to write an importer or if they have looked at the DB structure of XF is pretty much an unanswerable question. At the end of the day...if another forum software wants your business it really is their issue to write an importer (unless you want to pay a dev who can make pretty much make anything within your budget).
Choices are free, actions take resources. It's good that you consider the future of your community...but you probably wont get any fruitful answers that will provide you with a sense of security in regards to your question. If it is something that really worries you, you should vet other software titles or possibly build a relationship of trust with a developer who will be available to you should it come down to you moving on to another software title.
