THAC0 is still that way that AC is calculated, except they reversed the formula so you add instead of subtract.
It's still the same function: roll d20, add modifier. The ONLY difference is that instead of trying to get below the AC, now you need to get above the AC to hit.
It's the same thing with combat.
A round of combat is 6 seconds. Within that 6 seconds EVERY SINGLE PARTICIPANT in combat gets to take ONE TURN. If there are 100 participants, ONE TURN for ALL PARTICIPANTS still equates to ONE SIX SECOND ROUND OF COMBAT.
All they did was swap the word ROUND and TURN for people who cannot SUBTRACT, but can only ADD.
Combat in D&D has NEVER CHANGED because D&D IS NOT A COMBAT SIMULATOR IT IS A COLLABORATIVE STORY TELLING GAME.
THE ENTIRE DIFFERENCE BETWEEN WARGAMES INC AND TSR WAS THE DIFFERENTIAL FOCUS ON COMBAT.
If you sit here and try to break up the semantics of a six second round and try to tell everyone that a turn does not last 6 seconds, YOU ARE BEING A DISINGENUOUS TROLL WHO IS ONLY HERE TO CAUSE FIGHTS AND ARGUMENTS AND DON'T ACTUALLY PLAY D&D. You are nothing but a rules lawyer who can't even read. The worst kind of player to have at a table.
I swear to god the only people who think otherwise are the few people who think D&D is a combat simulator.
When you read a book, does the action happen turn based? When you read the Dark Elf Trilogy - was it turn based? When Wies and Hickman ran Dragonlance, it was turn based, but were the books? No. Because the turn based combat is an abstraction of real-time events. Characters in D&D don't stand around waiting their turn to take action. You don't need turns in a video game where that abstraction can be handled by the internal game clock.
D&D is not a combat simulator, it is a collaborative story telling game.
Alright, I'm gonna do my best to break down what you've said so I can address it piece by piece.
Yes, THAC0 is identical in practice to what we have now, this is a well known fact, and perhaps used a bit too freely to say that D&D hasn't changed when it most certainly has. Combat in particular has changed a great deal over the course of D&D's history, which brings me to something else you've said.
The thing is, though, you're quite wrong - and so was I in agreeing with something I shouldn't have. Combat did not even work in the same timeframe in AD&D. I had forgotten something crucial; A single turn in AD&D is ten rounds, and a single round in AD&D is one minute. The rest of what I said applies, still - initiative was an entirely different beast back then. I haven't played much AD&D, but I have indeed played it. EDIT: I am still unsure about this, because when I played we really did just treat it similarly to 3.5e in combat. The point here is just that combat has changed a great deal, and while the six second standard is well founded, combat in 2e at the end of the day worked better as a RTWP system than 5e does. I wish I remembered more about AD&D firsthand, or had the books on hand.
It's funny that you bring up the difference between Wargames and TSR... because D&D had its roots in wargames.
You'll see a lot of argument about whether or not D&D is a combat simulator that people find ways to RP around, or if it's a proper collaborative storytelling system like SCUP.
All of this said, I do agree that a video game does not need to be turn-based to be fun or even true to the D&D roots, but for 5e, turn-based makes a lot of sense. Reactions alone make a RTWP system seem unwieldy for the action-economy-centric version of D&D.