I am concerned that you are concerned about the possibility that Tesla has not done the necessary engineering to validate the Model 3 battery pack adequately and is offering an 8-year 100,000 or 120,000 mile battery warranty just because they are hoping the pack will be okay after that much time and miles.
Tesla now has billions of miles of data on the S/X packs and they are holding up remarkably well, in fact far better than many naysayers and self-proclaimed "experts" predicted they would. The Model 3 cells are not fundamentally different from the S/X cells and the BMS must be basically the same.
Tesla has proven that their EV battery technology is the best in the world. It's only going to keep improving.
Oh, the Model 3 packs will hold up just fine. The only thing I'd be concerned about is having them spring things on us like throttling supercharging rates after a firmware update or reducing the power delivery after a certain number of launches. These things are totally not cool and I would be upset if my vehicle were affected.