If you don't have anything to actually add to the discussion but edition warring at shots and 5th Edition, you really don't need to post. Attacking the game just makes people defensive.
Do you have any actual suggestions to offer on making 5e harder? Things we as the community can add and make? Would you like to contribute anything?
I have contributed plenty. Reining in feats, discussing how to combat archery supremacy, you name it.
As for the purposes of this thread, however, I just wanted to agree with you. Reissuing the Monster Manual (in an "advanced" version, rather than actually replacing the one we've got) would go a long way.
Terrain could also work, but only with better support. A DM needs to be able to get everything about an encounter with a monster from its MM page. Lair actions and descriptions is a good step in this direction, but of course they're almost always just ribbon features that really doesn't increase the heroes' challenge.
If you're* talking about "a monster can always be made challenging, given terrain exotic enough" that might be true, but it's an argument along the "this critter has an Int 20, it must play smart".
If it isn't in the stat block, it isn't there for the challenge rating.
*) you in general, not speaking directly or solely to you Jester.
That is, I don't care if the dragon or lich is an idiot or a mastermind. It has six ability scores. None of them matter outside of what's in the stat block. If its CR of 20 really is more of a 12, then there's something seriously wrong. That the monster COULD bring in traps and schemes and adds is completely beside the issue. If the monster is meant to be encountered with all that,
add it to the stat block.
Other than that, if the OP wants to talk about XP budgets and encounter frequencies, I will leave you to it. I don't see any future in having that as a solution, since there's no real science or accuracy to it anyway. I'd much rather have monsters that can challenge a fresh party right off the bat, and ideally on their own if that's their legendary role. And then just vary their power to adjust for worn-down heroes.
So yes, I agree with you. The monsters are key. It's vital to the game you don't have to use monsters 10-15 CR steps higher than the APL, because that doesn't really work too well - they generally can one-shot a hero. It does mean the fight becomes tense and exciting, because the party will per definition not be able to down the monster before it can seriously threaten them in return, but it sells hero resilience and stamina short.
I'd much rather have support for Solo creatures, where a CR 10 Solo generally simply can't be brought down in less than, say, three rounds no matter what*. On the other hand, its many and varied attacks are geared to not easily be focusable, and so it can in return not easily kill off a level 10 character in a single round.
*) by a level 8-10 party of five, that is
The end result: a mechanic that assumes that this combat - all on its own - is supposed to be exciting enough. No further adventuring that day assumed.
THEN, if you want to turn this encounter from "hard" to "double deadly" (or whatever), you can either add in warm-up fights (where the party is wise to conserve resources) or even adds to the main event itself (though it's okay if the book cautions you against doing so - even with the current rules, a walk-over fight against, say a Balor, becomes significantly more dangerous if you add a couple of much-lower-CRs adds, like half a dozen Barlgura guards or a pair of Marilith wives)
Hope you find this reply sufficiently expanded.