I will share my own experience with what (presumably) was an eMMC failure.
By January of this year, I was experiencing garbage icons in audio apps, after a firmware update, the garbage icons actually stopped showing up. but there was a new problem. ODO reading (50k km - about 31k miles).
The LTE bar would drop to one (even when my phone showed 4-5) then the audio would freeze at least once a day in random geographic locations in random songs. It would recover within 2 minutes. The first report I placed around february booked a mobile technician visit in about a week. I received instructions to log each time I see a freeze within a couple of days, so I did exactly that for the next two days. I logged about 8 timestamps of freezing audio within a 48 hour period and the remote technician said : "Thanks, that's enough, we have enough to debug. Please don't send any more logs".
During the visit, the mobile technician opened the side mirror cover to check the LTE antenna --- they were fine. The final prognosis : it was just poor cell reception that caused the failures. There's nothing he can do --- recommendation : use the spotify app on phone and bluetooth to the car. I decided to live with the daily hangs in spotify as a quirk of the privillege of joining the longest most luxurious beta test of a technological marvel. (Oh what fun it was to be in a never ending beta test of an amazing toy).
During the covid-19 lockdown, there was only minimal travel for groceries and occasional food pickup, very little distance travelled in almost 2 months. Then last week, I noticed garbage icon in a spotify bar and garbage in a tile of a navigation map refresh. Then the Energy readings grew faulty and showed 0 km in the (last 50km wH/km). When the car was parked, I did a soft reboot (press both job wheels) until MCU goes dark. This time the MCU stayed dark. And it stayed black overnight, the next morning. It never came back on.
The service call was made and it was fast. I got a loaner model S. The drop off was contactless (because of new covid-19 protocols). Within a few hours, I had a first message that the MCU could not be brought up and a new one will arrive in about 7 days), then a second message the next day said they have an MCU and will replace. Within 48 hours of dropping off the car, the car was ready for pickup.
The pickup was also contactless, but there was a glitch as the technician forgot to reset or enable something and the doors would not unlock via the tesla app. It did not take long (about 30 minutes) to rectify this - but I had to wave through a window and find someone to take a look at it.
Overall it was a lot less problematic than I thought. Driving without the MCU was a hassle as the dashboard display switched from km to miles and there was no AC - windows still worked.
Note that with the replacement, all the settings were wiped out. The car's name, the homelink programming, all saved settings and addresses (driver profile, bluetooth, spotify account, home, office, good food), all gone. So spent some time to reprogram them all again.
With the replacement MCU, I hope the eMMC lasts a lot longer than the last one. What's the experience of others who had their MCU replaced? How long before it goes bad again? Presumably the new firmwares disabled the incessant logging of data and we'll get many more years of use out of the flash memory?