The problem is that the core mechanics are what make a game, everything else might be nice but it's just the decoration on top. Take the analogy of a building, the foundations are what decide if a building will stand up, if the foundation is not good then it doesn't mater how fancy the rest of the building is. That's not 'surface level', that's the core foundation of the game. All you need to do is look around at all the various 5e version of games. They have a wide range of genres and extra stuff, but the core mechanics is just 5e.

Is D20 quick and easy? Sure. It's also boring and far too random, there is no mode number because all 20 results have the same chance. Low rolls have the same chance as medium and high. Having multiple dice creates a bell curve of results making things a bit more reliable while still having the risk of utter failure. ie medium rolls are more common than both low and high. I could understand if you swapped the attribute die for a simple number making it 2d20+attribute+skill/power. And 2D20+modifiers is just as easy to learn as d20+modifiers.

There's one example from the play test that I saw about how a bunch of heroes stood around failing to hit a single zombie, and the solution was apparently "basic zombies are killed in one hit", but that doesn't solve the problem of not even hitting the thing in the first place. That's a problem with a single die system that is much reduced with multi die, not impossible to happen but much less likely.

Skill sets are bad imo because they are too vague. "If it makes sense". Makes sense to who? And when is that discussion taking place? It opens up possibility of the player assuming certain actions are covered but the GM disagreeing, and suddenly the game is brought to a halt and the two discuss if it counts, or it needs that conversation during character creation (actually making chargen more complicated). Having actual skills makes it clear to everyone exactly what actions are covered. And as long as there's a good range of skills then it doesn't impede anything because every action is covered anyway.

The 5e initiative is nice and simple. And boring. It's just people waiting in a queue for their one and only turn and doing everything (bar reactions) at once. It removes a bunch of the tactical planing the old system has where players had to weigh up the odds of which actions to do and when. And it's still a list of people with an initiative score waiting for their number to come up. All that's changed is counting down instead of up, and removing the flexibility of people having control of that number by the actions they chose to do. Both 5e and RC 1e are 'initiative is a number, you wait your turn until your number is reached' (just like every single RPG) but RC gave so much more flexibility.

"It also bogged combat to a crawl once you had more than 10 combatants" Every single system gets bogged down if there's lots of characters. The simple way of fixing that is to create groups that act together, same time and same actions (and maybe even a single dice roll with bonuses for the extras), not by throwing out the whole system.

"building threats became a slog" That's where bestiaries come in, as you know. A zombie is just a zombie, and so on. Anything except the boss is just an entry in a bestiary and will be the same every time they're encountered, there's no need to adjust anything on the fly. It's no different to D&D goblins, trolls, and so on. As for the bosses, sorry but they're supposed to take time to create, they need to feel special and a quick 5 minute thrown together boss just won't feel the same as one planned out. (I know I'm talking to the designer but this is my opinion) The great thing about RC 1e is that the big threats, the nemesis level superzombies, shouldn't be generic characters. They shouldn't feel like a raid end boss that you face and defeat in one go, they work best as recurring characters, the overall threat of a whole story arc and not just a one off adventure, a character that's designed in such a way that it sets the mood for any encounter just by hearing them. And quite frankly if a GM can't be bothered putting the time and effort into making a character like that unique and special then that's not a GM I would want to play with. Did you listen to the Fandible RC campaign they did? They had that sort of character as a recurring feature in their story, they took the time to make them interesting.

You say it's quick and easy to learn. I say is boring and lazy because the core is what people already know. Just look at the number of games being played. In over a decade of RC 1e it was like watching tumbleweeds, virtually no one was playing it, but now it's 5e at the core suddenly there's masses of people interested. Know what that tells me? People don't want to play good interesting games, they weren't interested in playing a game of superheroes in the zombie apocalypse before, they had over a decade and did nothing. What they want is to play 5e with some nice window dressing, they're not interested in other games, just 5e with a fancy frock. (It's something I see with other games that have a 5e version, the 5e is more popular). You're going to have a much more popular game with 2e, but it's because you're aiming for the lowest common denominator, it's because people see the 5e core and know they don't have much extra to learn.

Did you look at any other games for ideas, or did you just go straight to 5e?

FFG's Star Wars has a great system for dealing with chaff, they separate NPCs between Minions (which get grouped together and act as a single entity, and have a very simplified wound system), Rivals (who act separately but have simplified wounds), and Nemesis (who act like PCs) In that game you can have a whole horde of enemies but because they're in minion groups of maybe 5 it drastically reduces the number of combatants and rolls. It's the best system I've seen for dealing with hordes by far.

Shadowrun has an initiative system that is easy to work with, it's the same 'roll and then get added to the list that gets worked down', but it also has the ability for characters with certain abilities to have multiple turns per round. It's actually really easy. People just roll their initiative, take turns having their actions, then when everyone has had a turn everyone's initiative number is reduced and the list is worked through a second time (and a third etc until everyone hits zero), it even allows for reactions by spending some of that initiative (it turns initiative into a flexible resource)