I apologize for my rambliness here.
Something I'm reminded of is Pokemon Red (I promise this is relevant). A Pokemon that shows up frequently when the game is corrupted is Rhydon, because Rhydon was the first Pokemon ever coded into the game. Also common is the ever-famous Missingno, because that's the default the game throws up when it knows it doesn't know what Pokemon to display, and third most-common is Kangaskhan, for reasons I don't really know but probably relate to it having a pre-evolution (almost certainly that little baby it carries around) that was cut last-minute due to the Game Boy's memory limitations. Similarly, one of the glitches is called the ZZAZZ glitch because it replaces most of the characters in the player character's name with a ridiculously long string of "ZZ" (the last letter in the alphabet) and "A" (the first letter in the alphabet) characters. Unless the name is the right length to ensure the end byte isn't among the replaced characters, this will render the save unusable because one of the ways the game checks for corruption is to see if the player character's name is within the character limit.
The relevance: Most "file corruption" cases where the save file stops loading come down to that. It's not that the save file is totally unreadable/unusable, but rather that the file has failed the checks that the software (or in some cases, the OS) has implemented to minimize the risk that it runs a program that causes permanent damage to or even outright bricks the software or hardware. It's annoying, but it's a very, very good thing overall. Many netizens (me included) aren't old enough to remember when tech companies either couldn't or wouldn't protect software and computers like that. So there's a tendency to take for granted that software can't hurt anything outside itself and that error message only mean that something isn't working (when they also mean that error prevention and containment are doing their jobs correctly). You may think you want your potato computer to risk running a large file, but actually, you don't (...unless you like the idea of converting it to a paperweight, I guess).
I'd love it if somebody had diagnostic tools so they can see which precise checks are failing and evaluate if anything from corrupted save files can be rescued. But in the meantime, this is why it's important to clear out the caches on a regular basis or at least after updates/mod installations (data stored in them is supposed to be temporary and is thus vulnerable to becoming obsolete, either because of changes over time or updates, and this can both gunk up the corruption checks with false positives and cause it to become corrupted in a way that's contagious to more permanent files), keep lots and families you're fond of in the gallery or library for retrieval in the event of file corruption, and get to know your computer's limits so you can keep your builds and families safely within them.
Also relevance: If I had to guess as to why Willow Creek is the first to fall, it'd be for the same reason Pokemon Red likes to fall back on Rhydon or Missingno. Willow Creek was likely the first world ever coded into the game. All of the default resolutions to stuff like null pointers are going to point to it because, as the first world ever implemented and one of three included in the base game, it will have the lowest values possible for IDs and things like that. It's also common for software to just move onto the identifier immediately above or below if one it's currently on doesn't work, so that's why the order would always be the same. If I had to guess, this also applies to lot types, and thanks to the game never really forgetting what a lot has been in the past, lots that start off as residential fall together whereas lots that started off as community lots go one by one. Magnolia Promenade is unusually large and I think not technically attached to any of the other neighborhoods (like it loads a separate area and stuff-- correct me if I'm wrong please, I don't normally play in Willow Creek!), so perhaps it has more anti-corruption countermeasures in place or is smarter about how it uses memory.
Alternatively, Sims 4 started life as an online game. Pure speculation, but perhaps the residential neighborhoods were originally meant to be far less complex, as "home base" lots that wouldn't be very large or expected to hold many Sims and might not even require the internet, whereas the community lots were for various types of private congregations and events, and Magnolia Promenade was a large public lounge. In that case, the residential neighborhoods would have been pushed to the limits of and maybe beyond what they were meant to do when the game was hastily transformed into what we know it as today. The community lots would have been under extra strain too but not as severely as the residential ones since they were meant for gatherings. And if anything, Magnolia Promenade would be under far less strain from not having to handle dozens of Sims moving around the place at any given time while thousands attempt to enter it. And the game never forgets what a lot used to be because, if the online game would be anything like the Sims Mobile, the lots wouldn't be expected to change much period, and something like type probably wouldn't change at all. And a microtransaction-heavy game would benefit greatly from being able to revert purchases easily because it would let EA avoid lawsuits/controversy if somebody's kid ran up a credit card with the game (this happens more often than you'd think with mobile games) and EA could also easily undo fraudulent purchases or accidental freebies this way.
Another thing about corrupted data is that it tends to produce ridiculously high values too. Like with Pokemon Red, the ZZAZZ glitch produces a lot of Zs. Also, if you use corruption glitches to trigger an encounter at the PokeMart, you'll get the "A hooked [Pokemon]" message because, when determining how you got the encounter, the highest possible index value corresponds to fishing. So game only looks to see if the determination value is greater than whatever index value fishing is set to, and with corruption the determination value is guaranteed to correspond to something ridiculously large. So I would actually wonder if the world associated with the most recent pack (either that you own or in general) tends to be affected more often too-- but this is harder to keep track of because while everybody who plays the Sims 4 has Willow Creek as the first world, which world you have as the last world will vary depending on what packs you own. The last world will also change as you purchase different packs, as new packs come out, and if/when you use the disable pack feature. Plus, more recent worlds have the benefit of actually being designed for the kind of game the Sims 4 is, as opposed to being cobbled together from a very different cancelled game.
Plus I'd be remiss if I didn't mention how this game's code is beyond mere spaghetti. It is basically alphabet soup for Cthulhu at this point. That'll invariably cause it to corrupt itself for reasons beyond mankind's capacity to perceive.
I hope you guys don't mind me ranting here, let me know if I'm off-base in any way. I just find this stuff fascinating.
