I never understood the THAC0 debate.
Everybody rolls a d20 when their character is attempting to hit an opponent – that won’t change. How you determine the difficulty of the roll is a technical aspect of the game that DMs should tackle while prepping their session. I never look at Armor Classes during a game: everything I need is on my cue cards. I fill one such card for each and every monster or NPC the player characters might possibly encounter during a game. Prepping is when I look at all the AC and To Hit matrixes. I check every possible combination.
For example, gargoyles. Four / five Hit Dice monster. Gargoyles hit the Dwarf (AC -1) on a roll of 16; they hit the bard or the ranger (both AC 0) on a 15; hit the cleric (AC 2) on a 13; hit the thief (AC 3) on a 12; hit the magic-user (AC 7) on a 8. The PCs hit those AC 5 gargoyles on a roll of 13 for the level 4 fighters (Dwarf, bard, ranger), 13 also for the level 5 cleric, 14 for the level 5 thief, and 16 for the level 4 magic-user. All that info goes right on the cue card.
Then you do that again for the grells and again for the wights and the carrion crawlers and the clay golem and the boss, whatever he / she is. When you run the actual game, you no longer need to bother with any Armor Class.
THAC0 or no THAC0, it’s all the same. Each character rolls a d20. Each monster rolls a d20. It’s been like that since 1974. If you do the prep work and do it well, then there is no difference between First Edition and Fifth Edition. There was never a real debate. It’s just how you tell your players what number they need to hit.
If anyone wants to hit me, they need 15.