Fair enough. I might disagree with your statement to some level, given that I think that from what we have seen of the 5e monsters; that they are easy to hit (weak) and their own attacking bonuses are not unreasonable, which in my mind, allows the martial characters some leeway for experimentation.
Characters get between 5 and 7 stat boosts.
As Stalker0 points out a couple of posts above, a +1 modifier on primary stat works out with a large bonus to effectiveness for martial characters so a player is highly incented to maximise that (2 stat bumps).
The magic system features hp thresholds, hp is generally good to have, and the same stat provides a saving throw that historically is most commonly faced (magic, poison, disease). So that stat is strongly incented to be bumped (2-4 stat bumps maximum).
Good saves need to be bumped to 20 if the PC is to hold his own in resistance. Rogues and Clerics are already covered here from their primary stat boost, above. Fighters are covered by the Con boost above, Wizards are strongly incented to boost Wisdom or face having no saving throw be at least 50% reliable.
Poor saves can never get as high as 50% reliability, but to have even a 25% chance of saving at high levels requires a +3 or 16 attribute. Players will either ignore the save and plan on failure with the occasional success becoming a pleasant surprise or will be incented to bump their poor saves as they can.
AC matters since the bounded accuracy affects incoming attacks too. For those not in heavy armour (Rogues, some Clerics, Wizards) Dex is an important part of the AC. It also happens to be one of the saves mentioned above and affects initiative. Rogues are covered by their primary stat boost above. Clerics and Wizards are incented to apply boosts here.
If the PCs get through that gauntlet of choice, any remaining bumps can be used off-trope.
We haven't seen very many finalised monsters. The only one I know of is the hobgoblin.
I would characterise a monster is easy to hit if the AC can be struck between 65-75% of the time by an optimised character or 30-40% by character with minimal combat ability. The range I suggest is actually greater than the range offered to low-level characters so let's look at both scenarios:
Minimal combat ability (+2 to-hit) 30-40% suggests a low AC is 15-17.
Maximum combat ability (+5 to-hit) 65-75% of the time suggests a low AC is 10-12.
The one example I am aware of (hobgoblin) is Challenge 1/2 and AC 18. AC 18 means the minimal combat character has a 25% chance of striking and a maximal combat character has a 40% chance of striking.
By 9th-level, the hobgoblin becomes easy to hit.
A minimal combat character will have +4 to strike and succeed in hitting the target 35% of the time.
An optimised character will have +9 to strike and now the hobgoblin approaches but is just outside the easy-to-strike category with a 60% chance of striking. If we consider only one strike a round landing sufficient, it enters the easy-to-hit category since the Fighter has a 84% chance of connecting with at least one of his two blows.