Re: Gallery
Well, in the case of the gallery plugin it looks like you don’t actually pay for the plugin, but for the access to it. Once you have it, you can do whatever you want with it, the code being (well, it should be) GPL. So, if dev stops (not likely, considering that Brajesh is making money off it), you could always fork it and keep the development going, involve the community to help out as well. Shouldn’t be too hard to make a plugin compatible…
If that doesn’t work out for some reason, then you still have all the data (the actual images and the database). You then would have to migrate everything to another plugin.