SACDs (and hence ISOs from SACDs) can have up to three “sections”, CD audio, SACD stereo, SACD multichannel. To be an SACD it has to have, well, SACD music. I know of no multichannel only SACDs, there are plenty of stereo only SACDs and also plenty of SACDs with no redbook layer.
When a SACD has both stereo and multichannel often (but not always) they have the same program. Some may have bonus tracks in stereo, multichannel or both.
Multichannel is always compressed with DST, typically at about 2:1. The stereo may also be compressed with DST.
If you are going to delete/erase the MC material then at one extreme you may go from individual tracks taking two times the space that the ISO uses (from a compressed stereo only iso, I don’t know why but they do exist) to the other extreme of individual tracks taking about 1/4 the ISO space for a disk which had uncompressed stereo and multichannel tracks.
Personally I like to keep my music in the original format, but I understand that people may have other concerns/issues.