that's sound advice for any monster meant to be a leader, rather than one of the mob, so to speak. It has precedence going back to day 1 as well. In AD&D for example, if you wanted to make a leader variant, you gave it better armor and up'd it's hit dice by one or two. Same thing can be done in 5e. Better armor and more HP and attack and damage are the simplest, and if you want a true leader, follow the leader/lair guidelines and give it legendary actions or even lair actions, depending on what you want. 5e gives a great structure of how to handle these things by examples of other creatures. for example, I like to make some of my older dragons spellcasters like in AD&D. if I ask myself how do I do that, 5e already provides a template with something like the mind flayer arcanist. "Oh, by giving it spellcasting ability like this, it's just a +1 CR. Easy."
Yeah, I think your approach is exactly right. It's consistent with the "old school" sensibility 5e strives for, and it's extremely easy. And I agree wholeheartedly that 5e gives us a ton of tools to quickly and easily advance/beef up monsters as needed. I think this thread is largely discussing a solution in search of a problem.
Just one minor point...
I'm fairly sure that the reason the Mind Flayer Arcanist has specifically +1 CR is not a rule in the sense that "spellcasting=+1CR" per se, but rather the ways in which the specific spells on its list can slightly nudge up it's offensive/defensive capabilities. I'll admit, I'm away from book right now, and Mind Flayers aren't in the SRD, so maybe I'm wrong... but this is what I found when I looked at other spellcasters. The NPC "Mage" for example, has a perfectly accurate CR when you compare his terrible defensive CR with the offensive CR of casting a cone of cold and 2 fireballs.
In general, CR is not really a representative of how difficult a monster is going to be to fight. I think it's mostly just an expression of whether or not the monster is able to do so much damage that a single lucky turn or two could start killing people (or has so much AC and HP that low level parties will struggle to bring him down in 5 rounds). That's why, right there in the DMG, they basically write off tons of things that add huge amounts of
versatility to a monster as being irrelevant to CR... like speed, stealth, teleportation, ranged attacks, etc.
Adding spellcasting is always going to dramatically increase a monster's versatility, when the monster has the time and the DM has the strategic insight to maximize that versatility. But that doesn't, necessarily, change CR at all. For example, if you give a dragon spellcasting, but it has little in the way of powerful, directly offensive/defensive spells prepared... I think it's CR would not actually change. The dragon already dishes out a lot of damage, has a lot of HP, etc. Chances are good the spells are adding versatility, but not pure damage-per-round.
Actually... okay this isn't such a minor point anymore. But the rest of this is also not a direct reply to you, Sacrosanct. Just more thoughts I have about this subject...
I think that this philosophy of 5e's is a big part of what people are complaining about. But I disagree for a simple reason: Not all parties are created equal.
A party of 5th level characters that think strategically and use every trick at their disposal will be orders of magnitude more effective than a party that rushes into every problem headlong. Even if, in all likelihood, these parties have comparable hit points, attack bonuses, and damage. In fact, in many cases, the aggregate party damage might even be lower on the more strategic group, since they will have more people spending turns engaging in battlefield control. Grappling, shoving, and hypnotic pattern all deal zero damage.
So, it absolutely stands to reason that there are going to be similar discrepancies between monsters. Some monsters have HP and deal damage. Some monsters have some more versatility in what they can do. An NPC Priest is CR 2, same as a Bandit Captain, and he can do things like maintain Sanctuary, Spirit Guardians, and Spiritual Weapon up simultaneously, and then throw out Guiding Bolts or Dispel Magic. Put him in charge of 8 bandits and see how he measures up to a Bandit Captain...
Some monsters are more threatening when you utilize intelligent tactics... some monsters have fewer ways to be easily buffed in that way. A Bandit Captain that divides his attacks evenly between the three melee attackers engaging him is actually
less threatening than a bog-standard Ogre. Much less able to drop or kill a level 1 or 2 PC in a single round.
There is really no way to account for all of this stuff in a reasonable way, as a game designer. The game will be played by hardcore wargaming tactical nerds, and ten year olds, and parents who just want to relax on Friday night with a beer and their friends. Sometimes, all three at the same table. To say nothing of role-playing reasons to make bad combat choices, of which there are many.
So CR does the best thing it can do. It mostly ignores all the versatility stuff, and just looks at the pure damage the monster can deal and take. By that metric, the ogre is just fine.