If we have enough diversity in enemy department, then this could work.
it could work at the beginning of the game. what about second half? you can't add easy monsters to hard encounters, so you will see bigger and bigger hordes of early game mid-bosses. how can you solve it? make speshul extra stronk monsters with unique look and skill set you can encounter only in nightmare difficulty mode of final dungeon i.e. 99,9% of players will never even see them?
This was already answered, but ok... Since you don't like this idea.
What's your proposition then?
Go ahead, throw in some ideas instead of just repeating the same point over and over again.
I'm not Reinhardt, but if we rephrase the question as "what's the alternative?" it was already answered too.
As the end goal is to make encounters harder not more tedius, there could be no universal one-size-fits-all solution.
If you automatically replace all mobs A with mobs B in every encounter, or provide game-wide +10% HP buff to each enemy it will be boring as shit.
The best result can only be achieved with dev's attention and manual tweaking.
And it would take all the options, from adding extra and/or special mobs, improving their skills and stats, to improving/changing enemies AI and limiting characters resoures.
This is why I like idea that it's better to design and balance the game around the hardest difficult first - lets call it's the "core rules".
It seems to be simplier to make things easier, not harder.
Drop an enemy here, equip worse gear there - and it will be guarantedly easier then the hardest difficulty without any extra attention to balance.