Doesn’t a good DM learn to say no?
Internal consistency is important.
Sorry this is a bit late, was combing back through the thread and felt I should reply.
In my experience, a good DM needs to learn how to say "yes" much more than they need to learn how to say "no."
Most folks, when they take up the DM seat, are not total rubes. They've played the game for a while before DMing. They know what it means to be a player searching for an advantage, or an exploit, depending on how one views the tactics used. Even if they don't do that sort of thing themselves, they've almost surely seen it from someone else. DM skepticism is alive and well,
particularly in this vaunted age of "DM empowerment" which basically just means "do whatever you want, whenever you want, for as long as you want, and give your players the proverbial middle finger if they don't like it; you aren't required to do anything more or different, and if players complain,
they're being toxic and horrible and ruining your beautiful game." Beyond that, we're still plagued by old favorites like DMs not understanding iterative probability (that old chestnut, e.g. "you have to roll Hide and Move Silently
every round you're inside," which merely guarantees eventual failure), or having deeply mistaken beliefs about what is physically possible IRL let alone in a fantasy setting where being angry enough can let you chokeslam dragons.
What DMs actually need--a skill they almost never
start with, but which is critical to develop--is learning how to say "yes"
well. Saying no is easy; you just shut down whatever the person is trying to do. Saying yes is
significantly more difficult, because there are grades of yes. No is no; but "yes" can be "yes, absolutely 100%", or "sure, but with this tweak", or "how about this, which does the same thing a different way?", or "yes, but it won't work the way you think", or "yes, and this other consequence too," or "yes, but it will take time/effort/resources/etc.", or...
The only variation on "no" is "no, but..." which is, IMO, actually a way of saying yes--because the "but" cashes out as, "well you can't have that very specific thing, but this is something I'm happy to say yes about which is close to what you wanted."
Learning how to say "yes, but..."/"yes, and..." is a crucial skill that many DMs take a very long time to develop. I find very, very few DMs need to learn how to say a flat "no", and as noted, "no, but..." is IMO actually saying "yes" in disguise.