> But still you might need to decode between 1 and N frames to get the frame you want, and you don't know how much memory it will consume...

All of these things are bounded for actual codecs. AV1 allows storing at most 8 reference frames. The sequence header will specify a maximum allowable resolution for any frame. The number of motion vectors is fixed once you know the resolution. Film grain requires only a single additional buffer. There are "levels" specified which ensure interoperability at common operating points (e.g., 4k) without even relying on the sequence header (you just reject sequences that fall outside the limits). Those are mostly intended for hardware, but there is no reason a software decoder could not take advantage of them. As long as codecs are designed to be implemented in hardware, this will be possible.