Even for all that is correct to criticize about Tesla's build quality, the batteries do hold up a while;

Even Tesla's warranty cites 70% capacity after 8 years / 120k miles, which roughly tracks with real world results.

Although I'd never buy one, the battery seems to be one of the least issue prone areas; usually people cite interior/exterior build quality, a total lack of serviceability and software issues as the main things when it comes to Teslas.