Yeah, the main reason was to introduce a mechanic that would make sense in this world and would increase the feeling of realism, even if we are playing a game where we fight demons and angels.
Maybe they could make somekind of invasions (like in wow) if a certain number of players lets say (50 for example) doesnt show up, the invasion (from hell or heaven) will destroy the encampment (loosing the townportal, vendors and crafters..) but if 50 or plus apear you can defeat the event and get a few rewars in the process, like the caches from dailys D3 events.
The capacity of loosing something would add more replayability to the game since it could always change from the last time you logged. Again as you said, in the D3 you just had to lvl up your crafters once and that was it, and a point here, defending the encampments should be hard, encouraging people to go there and now just thinkign, nah, a few guys can handle it.
Unfortunately i havent seen anything yet in this direction of degreadable encamptments, just making them similiar to crafters in D3 witch i think is a major waste of oportunity. The courrier sistem would help lvl this encampments more faster and rebuild to after an invasion, well it could even influence the itens in vendors tabs.
About the courrier mechanic, like you said many people would find somekind of meta for this, but as always this would be a very small number compared to the whole comunity. This mechanic would be just another way to encourage people to explore more the maps of sanctuary, valuing even more the work they are imputing in this game,and avoiding the current system in D3, just using TPs from one point to another.
At least the part from mounts items looks they are going for it, hope they do something close to the witcher 3 as i discriberd before, but i agreed with you about the rewards from this kind of quests, not becoming something indispensable but not to simple too.
Regarding the Ancestral (demonic and angelic powers too) they are really giving almost no info in the lasts development, at leat i hope they make something like i said in pvp (a pandamonium map) where you could choose a side (heaven or hell) and farm this powers to unlock the bonuses from itens or/and upgrade skills to higher lvls (giving new skins and effects to them would we awsome, but o'm dreaming here lol).