You're mostly wrong; MST does not increase bandwidth, it splits the bandwidth of a single DisplayPort link. To increase raw bandwidth for 5k/6k they combine multiple HBR2/HBR3 links (over Thunderbolt or with multiple cables), which is the opposite of MST.
MST is supported specifically for early 4k displays that had scalers that couldn't handle 4k60, but could handle half the resolution, so they sent two streams. But that wasn't a bandwidth limitation; old MST 4k displays and modern SST 4k displays both used a single HBR2 link.
MST is supported specifically for early 4k displays that had scalers that couldn't handle 4k60, but could handle half the resolution, so they sent two streams. But that wasn't a bandwidth limitation; old MST 4k displays and modern SST 4k displays both used a single HBR2 link.