Loudness-wars compression is a product of the digital recording era, and quite a few years into it, too. A lot of music that people still want to listen to today predates all that.

Many SACD reissues do go back to the source tape. This is a frequent cause of complaint with SACD reissues of classic jazz recordings from the 1960s: sometimes you get better sound in terms of dynamic range than any previous CD issue of that recording, but in the meantime the source tape may have deteriorated in parts.

Even with recordings from the “loudness wars”, there is sometimes room for dynamic range improvement when remastering. A good example is Rush’s album Vapor Trails. This was an infamously botched recording upon its original release, on a scale with Death Magnetic. Because loudness-wars treatment plagued the original tracks before mixing, the damage could never entirely be repaired. However, the additional process of compression applied to the source during transfer to CD could be reversed, so the album was eventually reissued as Vapor Trails Remixed, and while still flawed, that reissue has a lot more room to breathe than the original CD release.

Why implausible?

The scope of work involved is too great to remix and remaster all the records. I also know that most indie records are not kept in their multitrack form as meticulously as big label acts.