Agreed, but, arguably, it is not the app which would trap you, but the file format.
If a static site generator took as input, say, a collection of markdown files, with maybe some metadata in the filenames and directory structure, along with a single plaintext configuration file, it would be easy to move between different solutions, so it would be less configurable, than, say Jekyll, but easier to move to something else. I do not know if such a thing exists at the moment, but would be interested if someone does.
If a static site generator took as input, say, a collection of markdown files, with maybe some metadata in the filenames and directory structure, along with a single plaintext configuration file, it would be easy to move between different solutions, so it would be less configurable, than, say Jekyll, but easier to move to something else. I do not know if such a thing exists at the moment, but would be interested if someone does.