In my experience, what you are saying is true enough for the lowest levels of the ‘apprentice tier’ when the DM tends to coddle the players.
But at higher levels − for more ‘verisimilitudinous’ play styles − heroes are likely to come across creatures whose levels diverge significantly from their own. The AC can vary significantly. Often enough, the advantage proves unhelpful.
Yes, correct. The need for a natural 18 or higher happens often enough (especially for skill checks), and the advantage mechanic becomes less helpful when one truly needs a bonus.
I am not sure what you might mean by "for more ‘verisimilitudinous’ play styles"
However, if we are playing D&D 5e using monsters from the monster manual the highest monster AC is the Tarrasque at 25 (most ancient dragons are 20-22).
At tier 4 level of play (levels 17-20), the proficiency is +6, stat is +5 and a magical weapon of +2 is probably on the low side. In any case this gives a total modifier of +11 to +13 and a target number of 12 ... where advantage is equivalent to +5.
However, you seem to be playing in a 'verisimilitudinous’ game where a to hit die roll of 18 is common.
With a to hit modifier of 11 to 13 this means that you commonly fight home brewed creatures with an AC of 29 to 31. However, we are NOT discussing the effect of advantage in a homebrew setting with creatures with extremely high AC that have been created for a specific game. We are discussing advantage in the context of D&D and the monsters supplied with the monster manual ... and in that case, advantage, even at tier 4 is the equivalent of +4 to +5 over the vast majority of typical ACs.
Let's turn to skills. A character proficient in a skill at tier 4 will have a minimum of +5 even if they have a -1 in the stat. This goes to a maximum of +11 without expertise and to a maximum of +17 with expertise (assuming no magic items).
Have to roll an 18 on a skill check depends on the character and their skills. If you aren't proficient ... what do you expect .. the DC was probably about 20? And advantage is still the equivalent of +2.5 in this edge case.
For a proficient character an 18 required die roll is at least a 23DC task.
For a proficient character with the appropriate stat an 18 required is at least a DC29 task
For a proficient character with stat and expertise an 18 required die roll is at least a DC35
Keep in mind that "impossible" starts at 30.
The point? If you are commonly encountering DC18 skill checks from proficient characters then the DC of the task is already past very difficult.
On the other hand, a difficult DC20 task for a proficient character is again into the range where advantage is +4 to +5. An impossible DC30 task for a character with expertise and the stat required gives a target number of 13 and AGAIN, advantage is the equivalent of +4 to +5.
Finally, the reason you seem to devalue advantage is because it does not guarantee success. I have rolled with advantage MANY times and still rolled a 1 and a 2 (or other low numbers) ... and failed miserably. However, confirmation bias does NOT change the math that in the vast majority of situations even with high AC and DC at tier 4 ... advantage is equivalent to +4 to +5.
... trying to figure out the target to hit number at which it is better to not use GWM/SS when you have advantage is a different and also interesting question. 
