Any service may disappear at any time. That is a fact!
They may be closed down, go bankrupt or suffer a sudden data loss that kills the service. It have all been seen and we will see it again. Sooner or later Mapillary will probably go away, but lets hope it is later rather than sooner.
So keep backup of your images as far as possible. I have 2.5 TB Mapillary images at home. When I soon get a 300 MBit fiber connection I plan to back them up to the cloud. I have also considered getting an “unlimited” website and put them up there - I just need to find one that accepts around 3 TB of data.