Regarding the myth, the closest thing I ever found was relating to recall obligations. See
Deciding to keep or trade-in 2011 Leaf @ 3 year ownership - Page 4 - My Nissan Leaf Forum.
I agree also about warranty obligations.
A few years ago, my insurance agent unprompted claimed that even some folks w/older Model S were having trouble getting parts (presumably for accident repairs) and had to get parts specially manufactured. I didn't press him for details. I didn't call him to chit chat about Teslas and it was during business hours, so I'm normally not able to take too much time away from my day job.
You're relatively new here. Numerous folks w/Model 3's have already required pack replacement under warranty (for whatever failed inside). I've pointed people to their posts here on TMC about them before. Numerous folks w/Model S have needed new packs in and out of 8 year warranty. That number is only increasing.
I don't find the odds too good that a 3 will still be on its original pack after 20 years. They might be decent for say 10 to 15 years w/at least 1 replacement during that time (e.g. get lucky and receive replacement packs before the 8 year warranty expires and that final pack lasts another few years). If the replacement pack is too costly, many will just dump the car.
I will agree with the rest.
Wish the poll had more choices.