In short - the d20 mechanic enables you to resolve everything. If everything you encounter becomes something you can interact with mechanically and assign a DC to, a widget, then you are no longer actually roleplaying in a fictious world. You are just interacting with the mechanics of a game with a thin veneer of fiction layered on top.
This is true iff you think that having the ability to interact with mechanically means you must interact with it mechanically.
I’ve played coherent games with flexible, (almost) universally-applicable core mechanisms since the 1980s. This is not a thing that is new to D20. D&D3 didn’t invent having coherent, flexible, universally-applicable core mechanisms. Weirdly enough we didn’t at any point devolve into just interacting with the mechanics of a game because, well, we understood what the point of the game was and just appreciated having a way to adjudicate things neutrally when we needed it.
So first error: assuming that because you can adjudicate almost everything with dice you must.
Old School: “I flash the barkeep my best smile, order a cup of ale and pay with a handsome tip and try to get him talking about the local rumours in a chatty friendly manner.”
DM considers the scene and factors in the fighter’s 14 charisma and decides that a good impression is made.
Now let me strip the rose glasses from this and give other alternative outcomes that I have actually seen in those sainted “Aulde Skhoole” days:
DM considers the scene and factors in that the player took the last slice of pizza and gets churlish. Bad impression is made on NPC.
New DM freezes as something he didn’t prepare for happens and spends a half-hour flipping desperately back and forth between the PH and the DMG to find out what to do next.
DM makes up a reaction mechanism on the spot without thinking it through, throws 2d6, has them come up snake-eyes and decides the barkeep goes berserk and tries to murder the PC.
And so on. Because, get this, DMs are human too and sometimes have brain farts where ideas belong and stupid things happen. Having rules that offer guidelines, even if you don’t actually roll for a situation (more on this later), can lessen those brain farts and increase reasonable outcomes.
D20: “I flash the barkeep my best smile, order a cup of ale and pay with a handsome tip and try to get him talking about the local rumours in a chatty friendly manner. Actually a Persuasion roll. I roll 12, +2 from Charisma and +2 from Proficiency, so 16.”
The DM gives another +2 for the handsome tip and decides 18 is good enough to make a good impression.
I have, as I’ve said, been playing with (non-D&D) systems that have consistent, universal game mechanisms since the 1980s. I have never, not even once had any but the newest, greenest, most inexperienced players of any game do what he says is normal here. (And new, green, inexperienced players do stupid things in any system, OSR or modern!)
Here’s a more common outcome in my experience. (YMMV naturally, and if it does, I’m so sorry you have terrible fellow players surrounding you!)
Player: “I flash the barkeep my best smile, order a cup of ale and pay with a handsome tip and try to get him talking about the local rumours in a chatty friendly manner.”
GM: …
OK, let’s break down the GM actions by things I have seen once again.
GM checks the player’s stats and skills, realizes that on a Persuasion roll he’ll succeed about 80% of the time anyway on a stressful task and, since this isn’t a stressful task, and since the barkeep earns money by literally being friends with as many people as possible, decides the barkeep reacts well and is open to talk.
GM insists on some actual in-character interaction and notes that the PC says something that is taboo in town. Asks for a skill role on local lore and, with its failure, decides that the gaffe happens and the barkeep clams up.
GM insists on some actual in-character interaction and notes that the PC says something that is taboo in town. Asks for a skill role on local lore and, with its success, sidebars the player and lets him know and gives him a chance to undo the action. As a result the barkeep is friendly and aids.
And, naturally, if it turns out that this situation is critical for some reason, I’ve also seen:
GM asks for a Persuasion roll against a target number.
See how in the first case that’s almost identical to the so-called “Old School” case, and how in that first case having all the tools to do the roll helped make the decision without, you know, the actual roll? See how in the second and third the ability to do task rolls on anything gets some nuance in the RP, even though the actual persuasion attempt wasn’t rolled out?
See how, in a case where it might be needed, the persuasion attempt could actually be rolled out in a way that is understood by everybody around the table instead of some poorly-thought-out ad-hoc thing?
So just to repeat this theme here: the fact that you can roll for almost any situation doesn’t mean you should or will.
And I think any sane person who has read to the end would now agree that the d20 mechanic should die in a fire. It was an interesting experiment. Maybe we are all better off for having tried it. But we are not better off for persisting with it.
I guess I’m insane, because having read to the end the only thing that I think needs to die in a fire is OSR grognards who denigrate other styles of play. Who preach BadWrongFun™ because people are having fun with something other than the games they wear such deeply rose-tinted glasses for.