Well, yes. I think that submission bias is implicit in the wishes of the submitter, as you said. Nobody is going to submit an 1884o Morgan with a hole in it, that is worn to a slug. Likewise, if someone has an 1894 o Morgan, with great luster, it is a definite no brainier ( unless the coin shows an obvious cleaning, that breaks the luster). People don't generally submit coins hoping to get a G 8 grade, unless it is a key date. Likewise, a clean, bright or beautifully toned MS looking coin is one begging for submission. So, you are correct in assuming that the submission process is self selecting.
Phasing out the Carson City mint, and as was said previously, only 5 known die pairs. Once the dies get that badly worn, DMPLs are unlikely to be produced. Also, the 1893cc is an exception to the maxim that all cc dollars are well-struck. I have seen some pretty poor strikes in that date.
I am not as familiar with the Morgan series as many of the posters in this thread, so I cannot comment on that. However, I will comment on toned PL's and DMPLs: if you like toning, then you owe it to yourself to buy a toned PL. An attractively toned PL, when put under the light, will make the toning flash like you've never seen before. It is truly beautiful.