Remember how much Sand Giants and Gryphons in Everquest added to the sense of "this world can kill you if you let it"?
I agree with you on this point, it's thrilling being in a world that's challenging in this way. However, your comparison is a tad flawed.
If I were to compare this to Everquest using your Sandgiant model I'd have to give the Sandgiant Invis and Harm Touch and make him spawn in random locations throughout the world. As most of the complaints are from offscreen invisible mechanics like Cold Traps etc. In a lot of cases, we don't quite have the ability to see the Sand Giant on the horizon. And thusly we cannot make strategic decisions in regards to the threat ahead aside from not being in the zone in the first place.
I think this model of "challenge" is inherrently flawed, post 50 if you don't stack the crap out of defensive nodes, you'll just be gambling away 2-4 days of your life by simply entering Merciless. And even if you do stack defensive nodes, you take one cold trap to the face, or get CB - Cycloned down at zone in.
As it stands, It's just completely punishing to even log into Invasion.
Solution: Keep these overtuned ass bosses inside the corrupted zones, up their loot potential, give them unique loot or whatever. This would let us make cognitive decisions as opposed to just subjecting us to RNG Oneshot GG.
Take these bosses out of the maps, instead, put an extra chance to spawn a corrupted zone in the maps, and let the corrupted zones incorporate the rolled map properties as well as adding their own. Giving enhanced reward for the risk.