I think that it's also short sighted. Take a world like eberron where you dn't have a tier 3+ retired adventurer poing as the baker butcher & shopkeeper in every other town & limit the scope to just khorvaire were there's not really
much of anything capable of challenging a level 20 anything. Under those conditions the only reason a level 20 would need to take over rather than using their power to participate in & enjoy the fruits of a productive reasonably stable advanced civilization is if that level 20 were criminally insane. Once you dive into that sort of crazy all of the convoluted schemes are likely to fall apart through incompetence in the face of the actual
chessmasters in the world. Even if the level 20 wanted to do something like horrific arcane experiments on people for whatever reason, taking over the world is an absurd amount of effort when it could be done quietly in a remote area or even with funding from a proto-megacorp like ta dragonmarked house or a functioning established nation state interested enough in results to keep prying eyes/legal complications away.
Faerun may lack nations & governance due to rule by a continual series of violent overthrow of the last absetee warlord dubbed adventurer but even a
crapsack world like
darksun has numerous powers wielding the clout of a functioning nationstate complete with functioning military/policing/etc or similar but parallel levels of power who would just give the level 20 a cushy job mostly consisting of doing the thing they wanted to take over the world for.
Due to bounded accuracy & the assumption of no feats no magic items, any level 20 has been a fully functional armed & equipped
T-X for
many levels even if not using those things in their build. That makes the level 20 bar just silly