Veteran 5E DM here, and veteran homebrewer and home designer. 5E is balanced around a 65% hit chance for "typical" equal CR foes. Most fights, though, are going to be against more numerous, lower CR foes, so that hit rate will be a little higher. So the base game is already keyed to a 2/3rds or so hit rate. Some monsters (like the 15 AC Goblin, who has a shield and won't get that AC when using a bow mind you) have higher AC than normal. Many monsters (animals, ogres, zombies) have lower AC than the baseline.
But, like many have pointed out, adjusting the hit rates to be "more fun" and "more reliable" will require them to go through and redial other aspects of the game, and I hope the designers are paying attention to that. If you "double" enemy HP, you'll need to "double" the amount of HP the sleep spell does. If you're dialing down AC, you may need to dial down saving throws too (I was noticing Sacred Flame getting saved against A LOT).
I'm just glad they didn't redial things so attacks always hit, like they had talked about. That would have been rough.
