Reaching high level hasn't been the success nor failing point of a D&D game. Further, these comparisons I always see to either older editions or official Netherese stuff from earlier editions don't make sense in the context of 5e. The ruleset changes, the lore does not - a 20th level character is still of incredible power relative to those around them, as before.
Not truth. Do you know that on the module " The Throne of Bloodstone" introduces a city called "city of liches"? Being able to cast 9th tier spells is just one step towards lichdoom.
As for rules X lore, in a good game the rules are aligned with the lore. This is why Mistra dies a lot. They need to re write the magic system a lot...
5E gives you all the freedom you need to make a setting, like Shadowfell, be as dangerous as you want/need it to be for your level. 5E is leaving more creative and imaginative freedom to the dungeon masters.
This is not truth. Did you played Oblivion? On high level, combat becomes tedius due the way that the damage/hp scales.
To pick an example, imagine two lv 1 warlocks. One warlock with eldritch blast CAN OHK another WLK with 8 hp in 20% of the time. However, a lv 20 warlock even maximizing the rolls can't 3hk another lv 20 warlock; each lv up, the damage goes up a little and hp a lot. Compare it to 2e where you gain almost no hp after lv 10 and CON bonuses are very tiny. Even deity avatars and ancient dragons rarely goes above 200hp.
Poisoned arrows are a really nasty stuff on 2e. On 5e, are a minor inconvenience.
Shadowfell on 5e will not have the same lethality than on 2e. Dark Sun too. Dark Sun also has the political aspect. In a world where even Orks are problematic, having all dark themes like slavery being treated in a game would not be good.
Even I who LOVES epic campaigns realize that 5e will just not work past lv 15. The difference is that for me is due "oblivion effect", not balance or ultra powerful spells. Because lets be real. Everything that a PC can cast, a NPC can cast. And is not just my opinion. Descent to Arvenus, an adventure which ends up on literally the Hell has your party going from lv 1 to 13. IDK any module for 5e which goes above 15th level. Pathfinder 1e is different. A lot of modules end up around lv 17.