Great post, but I'm going to take exception to this bit. The game was designed to work out of the box, and they did a lot of work trying to get the balance right. That's not to say it didn't admit of modifications. We heard earlier in this thread that there was a last minute change to monster HP on initial release that made combat into more of a slog. A common mod to 4E monsters I remember for folks who wanted combat quicker was to reduce monster HP and increase damage, and we saw that monster math got fixed/altered in later monster books after the initial MM. We also know that the initial Skill Challenge math was a bit borked due to rushed development/release, and that needed to be corrected later.
4E was open to house rules. It just didn't NEED them as much as other editions, maybe.
Well, there are different ways to look at this. I do think it is an overstatement to say that 4e didn't allow modification. But it wasn't
quite as house-rule friendly as prior editions (or 5e).
Let me explain my reasoning behind this.
Think of two game systems. I'll use OD&D (LBBs) and BiTD (for those less familiar with acronyms, that's the original three "Little Brown Books" of the Original Dungeon & Dragons, and Blades in the Dark).
OD&D, famously, could not be played out of the box. You
had to "mod" it somewhat, even to get it to work. Whether through 3PP or magazine articles or your own house rules (and procedures of play, aka the infamous DM binder) ... it didn't just allow for modification, it required modification. It was less complete game, than a toolbox for creating a game.
BiTD, on the other hand, is heavily prescribed in terms of the setting, rules, and processes of play (heuristics and the like). That doesn't mean you can't mod it- people to propose custom rules. But it is very uninviting in terms of substantive modifications or house rules, because the game is "tight" in the sense that it has been designed as a whole to function together. In fact, if you start to seriously "mod it," (in terms of settings, rules, etc.), you're probably better off just going to a different game based on the FiTD system.
Which brings me to 4e. It's not that you cannot houserule or modify it; the 4e DMG (which I am assured people actually read, unlike the 5e DMG!) has a DM's toolbox section which include ... wait for it ...
Creating House Rules. But it's one page, and it just has an example of a fumble, and critical success and failure.
The issue is that the very advantages of 4e- the balance, the "tightness," the reasons that 4e is appealing to many people ... those are the same reasons that make it much less attractive to house rule or modify. There is no free lunch when it comes to design decisions, and the aspect of the design of 4e that are great also mean that it is less amenable to modification than some other versions of D&D.
IMO, YMMV, etc.