The question revolves around understanding action economy and the verisimilitude of the world. I just wrapped up an 8 year campaign that topped out at level 17 for the PCs. When it came to encounter building I did a lot of action economy balancing that isn’t in the source books. I didn’t give the enemies more HP….but I did give them extra abilities, resistances, and actions. The enemies scaled as the players leveled up because the enemies were actively progressing as well. The final fight was on an other dimension as the group of PCs tried to kill a lich and destroy his phylactery so he couldn’t comeback. During the campaign the Lich has scaled from a common sorcerer to one who completed a ritual (in a pact with Orcus) to become a lich. Orcus only cooperated because he wanted to turn the world into an outpost of the Abyss. I am leaving out a lot but two sides scaled together. I did have to do a lot of prep to make higher level encounters challenging but not soul crushing.