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.