Actually, there is. There is mapillary (bought by Meta at some point) and KartaView (I don’t think it is that popular). Code is not free, but licence for images is permissive. My personal favorite is Panoramax - completely open source, open licence and even federated!, but it is still in early stages, but keep an eye on it, looks promising. As always, problem with these efforts is not source code, but rather data.
not sure why you think this? You still have to have some state (you cannot just rehydrate state of file system upon restart and keep everything in mem). To rephrase, those who don’t understand databases are bound to reimplement them…poorly. Why you think upgrade of metadata schema in those files will be less of an issue on upgrades (surely this will happen, file format will change, just now without constraints, foreign keys, checks and with manual reindexing and manual query optimizations)?