@diopter77 I checked for myself on an EA App install I rarely use, and one where I hadn't ever signed into the game either through the launcher or at the Main Menu, at least not with that user folder. Inside Downloads, I only had my downloaded items plus FSIViewCountTracking and nav.bin. When I signed in at the Main Menu, I saw a bunch of badge-related files, plus [my email].bin. I never saw an MSI.bin.
Interestingly, wl.bin wasn't present, and also isn't present in a different test user folder from August, where I also wouldn't have signed in. And the file wasn't created when I signed in either, but it did get created when I exited the game a minute later, or possibly when I opened the launcher immediately after that. And after I deleted the Downloads folder and opened the launcher again (without signing in), wl.bin showed up again, along with nav.bin but nothing else. So it appears that the game stores the record of my having signed in somewhere, or some related data, and recreates wl.bin thereafter.
This is kind of a moot point though, at least as far as your issue is concerned. This folder I used to test has very little launcher-installed content, but that content did load in-game. What I think here is that your launcher-installed content is no longer recognized for some other reason unrelated to the files in Downloads. To try to fix it, please remove the SigsCache folder inside the main Sims 3 folder.
If that doesn't help, please use Method by Mell in this article:
https://sims3.crinrict.com/en/2011/01/tutorial-how-to-get-rid-of-unwanted-custom-content.html
to test each of your .dbc and .ebc files in DCCache individually. (You can test one of each at a time; they don't affect each other.) The idea here is that the game stops reading all of the .dbc or .ebc files as soon as it encounters one that's corrupt. So it's possible some of the files may be intact, and the game would be able to use them if the unreadable files weren't getting in the way. If so, you wouldn't need to reinstall everything, just the items bundled into the corrupt files.
If all .dbc and .ebc files are corrupt, you'll need to delete them and reinstall your existing content through the launcher, unfortunately.