Hello HaneulFier! Thank You for the additional info on this problem. In my case, although I have alot of Sims in my save file, when this problem started for me, I only utilized about 30% of the lots available in the game for builds and customization and my problem started pretty soon after installing For Rent. I had all of the packs until then but there were 3 worlds that I never even touched. You are right, the problem is complex and I honestly at this point don't think it will ever be fixed because it may be easier for EA/Maxis just to make a new game unless there is a fix they can implement in the code somehow.
However, with all of the days of experimentation and testing that I have done, there is a direct relationship with the # of objects (polygons) that you add to the game and this error being triggered although this is not the only trigger. However, with all of the variables that everyone has with differences in PC hardware/software, platform, mods, game versions, or etc. I will add the "disclaimer" that how my game behaves in respect to "objects or polygons" is definitely not what could be happening in anyone else's game to trigger save corruption. I still play the game often and I rarely experience crashes but if I made a YouTube video, I could easily show how I can add smaller builds to empty lots and be OK and then replace those builds on the exact same lots with just the right amount of build objects and immediately trigger save corruption. I can also show this if I added the latest packs (Businesses & Hobbies and Enchanted by Nature) to my game. This is why I can not purchase these packs. I can also show, that if I did purchase and add these packs, the tedious steps that I would have to go through in order to add the packs and get rid of save corruption, basically deleting enough of my current builds in order to make room in my game to add the packs. (# of Sims and households would have no effect for me.) Adding Sims, having large save files which I have always had, adding tons of mods which I no longer have except for the ones that I actually need, etc. will not trigger save corruption in my game but again, everyone's game has it's own nuances that can cause some to experience save corruption for different reasons and others to have never experienced it, "yet".
I will note that there are some players who are playing the game and their save files may be already corrupted and they probably just don't know it because warning signs are very obscure now that changes have been made to the game so that certain warning signs and codes no longer appear to indicate that there is a problem. The only way that I can know whether or not my game is OK is to check the community lots in Willow Creek "in build mode" but for some players, the problem starts in other worlds. This is a very tedious process so I am sure that many just will not take the time to do this each time before they save their game. Frequent game crashes or not being able to replace or delete random lots in any of the worlds are some of the signs that a save file could already be corrupted and I use to have these problems more frequently but I rarely experience game crashes now that I have implemented work arounds in my game to minimize the risk of save corruption and I don't add any new worlds that come with new packs.