That's not a good DM, that's a terrible DM who runs Schrodinger's Dragon, who not only has variant spellcasting but also conveniently has every spell in the book memorized even when it's too high a level (need CR 21 for Etherealness so even CR 17 adult reds don't qualify, only ancients do) in spite of the fact that dragons only get a small handful of spells (Cha mod I think) and don't have any way to detect that someone Ethereal is in the vicinity. Metagaming and stat-boosting by fiat aren't related to "good tactics" or good DMing.
1) He was discussing 17+ level PCs, so they would not be fighting Adult Dragons, but Ancient ones. Adult dragons, even ones who could cast spells, would tend to be easy encounter for PCs 17th level and higher.
2) Every edition of D&D had dragons that could cast spells. Just because the MM calls it a variant does not mean that it cannot be used. In fact, I would rarely use one that could not cast spells. Dragons without spells are just big flying brutes with a lot of hit points and a breath weapon. Meh.
3) It's not that the Dragon detects someone Ethereal. It's that the scenario presented was one of the party arcane caster was casting a 9th level Foresight spell, and a 7th or higher level Etherealness spell, and the PCs basically knew where the Dragon was located, and they were able to shoot at it from far range, and a bunch of other conditions that favored the PCs. If I were DM, a dragon would have spies for dozens of miles around. Loyal minions who would report strangers, especially strangers that wear armor, have weapons, and cast spells. He would have traps and possibly a fake lair and guardians and magic items and a variety of nasty unexpected stuff.
But I do think that there are some spells that a Dragon might be likely to have. Feather Fall. Dispel Magic. Shield. Mirror Image. And for ancient dragons, Etherealness is not out of the question (but, Teleport might be a good alternative).
As DM, I do not adjust an encounter on the fly and throw out the best spell I can think of. But, I do adjust encounters based on the audience (i.e. the players and their PCs). A group of noob players? No, they do not get super tough encounters. A group of experienced players with super optimized PCs and tactics? You bet. I throw the kitchen sink at them. The will not enjoy an easy encounter and will want to flex their tactical and ability muscles.
I do however agree that a smart dragon won't let the PCs choose their battles. I think a smart dragon won't wait however until after the PCs have already raided him. Instead, he's got his hooks in all the humanoid/human tribes in the area, has spies in civilized areas, etc., to proactively seek out threats, and he is perfectly willing to destroy enemy humans while they are in the middle of their nightly long rest. (Note to self: ask my players when the last was their PCs removed their boots and armor and took a bath. Surely they must take it off sometimes?)
It still requires the spellcasting variant to make this work well though. Vanilla dragons are pretty limited... And brittle.
Not if the dragon has surprise and allies on his side. But yeah, to me, the game is DUNGEONS AND DRAGONS. Not DUNGEONS and dragons.
In fact in my game, I double the Charisma modifier number of spells a dragon knows to make it more in line with earlier versions of the game. The toughest 3E dragons could cast over 50 spells per day. I don't think 5E dragons should be that capable, but having 2 to 10 spells per day when Eldritch Knights get 11 a day max is not unreasonable.
Some of the earlier version dragons were also extremely good at grappling, something that got lost in 5E as well.
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