I'm aware of all the "archival" issues. My oldest drymounted prints go back 40 years. I did not have a press then so I used a steam iron. (no water)
Still mounted flat and firm today. The only time I had lifting problems was when I used foam core. I've since learned to mount with foam core but try to avoid it anyway. Now Mr. Weston was know to use some pretty cheap cardboard. Nobody seems to be turning them down though. I like the look of a flat mounted print so I do it.
I think there's an awful lot of ego involved in thinking that museums would prefer an unmounted print for archival purpose.