Just rip any number of 80's CDs and normalize them to 100% and you will likely note that there is quite a bit of gain necessary to bring them up to 0dBFS. The further you are from 0dBFS, the lower the effective resolution is.

I don't think that's the way it works. Gain != resolution. And in my experience, the discs in my CD collection, including the old ones from the 80's, are all within a few percent of 0db. There are exceptions, but they are rare.

Even if the mastering hardware didn't have as much true resolution as 16 bits would tend to indicate, they would still adjust the compression and gain correctly.