My late father was site manager when the NLS building was substantially extended (or was it all new, I can't remember), Obviously as well as running a building site he had quite a lot of contact with the library management, and came away with prints of various old maps, 6 inch and 25 inch, or the area where they lived at the time, Kincardine, Alloa etc. I still have these.My old man was interested in mining, but did not do much underground stuff. He got the prints for free of course. It is good to see that it is on-line now, and anyone can have a look.
I am not sure if NLS have an absolutely complete set of maps, there are some parts that we may never see.
If Old-Maps dies out, I for one will not feel any sympathy for them. They actually sell stuff which, in original form, is long out of copyright, and put a fresh 70 year commercial copyright on it. That is quite objectionable, because they are not making a new "arrangement", which in all fairness can and does attract a new copyright, but merely reproducing a scan. Crown Copyright lasts for 50 years, from the end of the year of publication, so all OS maps, in original form, dating from 1965 or before, can now be copied freely. But only if you have an original.