I don't think this is quite right. If you serve an mp4 with H.264 video statically on any basic webserver it will just work in browser through plain HTTP, with

I don't think this is quite right. If you serve an mp4 with H.264 video statically on any basic webserver it will just work in browser through plain HTTP, without need need for MPEG-DASH/HLS. Every widely used mediaplayer/browser just downloads the nearest chunk (keyframe point) behind the time that was seeked to can resume the playback from there. This point is found through an index stored in a container format. For basically every video format these days (say, at least as new as H.264), regular settings make this only a few more seconds of video to download and decode before the seeked point and basically happens instantly for normal online consumption. In H.264 forward prediction (through two-pass encoding) will playback fine too. I think what you're saying applies more to a setting where the video is being streamed live, so that you cannot access the start of the file to get keyframe metadata. In that case HLS and MPEG-DASH help.