My oh my this is an old old thread!!
But I wanted to chip in because I had exactly the same question. (And in Cubase 9 it’s still the same)
Whilst it’s true that you can’t lock the mix position (I take this to mean the mixer view - not locking all the faders, but I don’t think that is what was meant)
What I’ve discovered I can use to more or less do this is the “Add Configuration” feature.
And very usefully any new configs that you save (which are project specific) become available on all three of the mixers although you can set three completely separate ones for each.
Annoyingly (well to me anyway) if I add a new track to my arranger window the new track instantly pops up on my mixer views and again screws with the view that I had set - but then just selecting the saved mixer configuration form the drop down instantly restores the version I wanted.
But I agree with the btrox (all those years ago) I wish I could just lock the mixer view and forget.