Here's my data point ...
June 2016 built MS90D, purchased new, owned now for just over 7 years. Original warranty coverage ended Nov 2019 when I passed 50k on the odometer. That says I've been running without the OEM factory warranty for over 3yr 8 mo. I likewise debated getting extended coverage, which at that time really meant the Tesla extended service agreement. I ultimately decided not to purchase that. Since that time my out-of-pocket warranty expenses are:
Rebuild/repair of two door handles (driver front and rear).
Replacement of the 12V battery
Replacement of the 3-way coolant valve / radiator bypass (coolant leak noticed from front of car)
Total out of pocket to date is $1,114. That splits out roughly at $450 for the coolant valve, $450 for the two door handles, and $200 for the battery.
Now for full disclosure, I did elect to do the MCU2 upgrade, but I do not consider that a maintenance repair and relative to the question of extended warranty coverage, would not have been included. I also had to have the windshield replaced last year due to stone impact damage causing a large crack to form. Again, I do not consider that to me a maintenance item, but if you want to include, total cost to me was the $100 deductible on my auto insurance.
So in my case, I chose to not get extended coverage and do not regret my decision. Your mileage may vary ...
Good luck on your decision and hope you continue to enjoy your MS.