I do think apps are useful for maintaining some level of reading fluency after establishing a baseline competency. I took 4 years of French through high school and college, but that was years ago. I stopped working on it for a long time and lost like 90% of my vocab and conjugations, but apps are bringing most of it back. They haven’t helped at all for listening though, I’ve been relying on listening to TV/movies/podcasts in French to improve as I can’t afford tutoring with any native speakers.