I largely don't agree with this. When you start getting into the higher levels, around 12, the CR system begins to break down. I know because I've run a game that ran from level 1 to 14, and by 14 you really need to start throwing some truly dangerous stuff to truly threaten players. Things listed as Hard in the CR system are actually Easy, and pretty much every fight they come across will need to be considered Deadly by the CR system (the designers probably know this too, as higher level adventures for 5E are not really balanced at all by the DMG CR system). It's even worse when you consider the magic items they may have, and by level 12 a party should have some pretty good ones (or the gold to buy them).