I think part of the "high level" nature of the original 1e modules was due to the fact that 10th-14th level PCs _were_ the highest level characters around at that point during D&D's early history.
Tomb of Horrors was first run at Origins I in 1975; the Lost Caverns of Tsojconth [sic] was run at WinterCon V in 1976; the G series was published in 1978. D&D was only 6 years old in 1978; at that time, Robliar, Mordenkainen, Drawmij, and others were among the highest level PCs going.
Such challenging adventures were also created for the original crowd who played OD&D: tactical- and strategic-savvy wargamers. They survived encounters that would kill higher level PCs now because of their skill as players: playing while everything in the game was still a new and unexpected challenge.
So, to more directly respond to your question, Merric, I'm not sure that the original "high level" range (10-14 for OD&D/1e) scales over time as the rules and challenges become more systematized/regular/predictable. I would think that as players grow more familiar with the rule (with demi-liches, for example), that the challenge of the high-level games would in fact fall vs. grow, unless the DM was to continue to introduce new elements that the players could not already be familiar with.