Yes. Yes to all of that. High dex, high Init, high mobility and a strong ranged game are how to spank 5e encounters. This was also true in 4e (perhaps less so) and 3rd. My memories of 2nd and 1st are less clear, but I don't think we started using a battlemat until 3rd.
The thing is, most players do not know this truth, so much so that I find it mildly thrilling to find it clearly stated here. They do not covet the Mobile feat. They do not build dex based melee combatants (except for rogues). But they also don't know where to stand on the battlefield. So maybe those naive designers are not so naive.
IME, most players are aware of much of that. They simply are less interested in "spanking encounters" and more interested in playing a character concept that resonates with them.
In that class survey that someone just released, battlemaster was ranked as the subclass that is most fun to play. And that doesn't surprise me. I played a melee battlemaster and it was a hoot. But I did retire him after level 11. I knew his best days were behind him. My fellow players were aghast. They loved my battlemaster. He rocked. I brought in a sorcerer spell sniper, my answer to tier 4. Take that, legendary resistance. The sorcerer was not as well regarded. But he was really efficient.
Case in point: your optimised sorcerer was much more mathematically superior to your battlemaster, so you probably enjoyed winning encounters with him. To the rest of your group, who regarded having fun as the way to win D&D, the battlemaster was probably more fun. I'm guessing partly in the way he played more probably more in the character interactions that had with him as an established group member.
Most of the melee weapons are STR based. The best armor requires STR. You have to go against the grain to build a DEX paladin, fighter or cleric. And if your DM uses dice to determine what kind of weapon that magic weapon really is, you're not going to get one. Is it worth doing anyway? Yeah, it is. But if you don't know that, you aren't going to bother trying. And people just don't know that. It's not in the book.
Its not really hidden: I've found that many are aware of good ways to eke out extra performance. For most however, its just not their highest priority. If they want to play a Dwarven strength-based melee combatant, then they'll play a Dwarven strength-based melee combatant, even though they know that a wood elf handcrossbow expert might be theoretically more optimal mathematically.
I would say your figures are two to three times lower than what well-built (call it min-maxed if you must) characters (in games with feats and multi-classing) reach.
Bear in mind I'm guessing that that is intended as a sustained figure for an entire party.
When my wizard threw a fireball into a werewolf pack, his damage for that round was much higher than 20. While he was hitting the survivors with his Chill Touch cantrip, his damage was considerably less.
Likewise, the Hasted, blessed, GWM fighter is likely going to be doing more damage than that, but those concentration buffs, and the time taken to cast them, are reducing the DPR of the cleric and wizard.
When considering party capabilities, particularly when evaluating their performance against a single climactic encounter, I think on-demand nova damage capability is probably more important than the sustainable party average though.
