Currently, Tesla's EV batteries have remarkable longevity. Everybody talks about replacing them, but you really don't need to. Tesla Roadsters (lithium-ion) and 1st-generation RAV4 EVs (NiMH) are still on the roads with their original battery packs and much less range degradation then expected, the 2nd generation RAV4 EV has an 8 year/80k warranty on its Tesla-manufactured batteries, the Model S has an 8 year/125k warranty on its batteries, and both are going strong after a couple of years and are expected to last substantially beyond their warranty. My understanding is that most people in the EV community expect these cars to be fairly easily able to go 10-15 years on their original batteries, albeit with reduced range.
I think we can reasonably expect the Tesla-produced EVs to provide reasonable range for over a decade and for 150k miles. After a decade or 150k miles, assuming electric vehicles will continue to be mass produced at the same rate they are now and are not rarities, resale value of these 10 to 15 year old cars will already be greatly reduced due to other factors, as they are with decade-old gas cars. There will be wear and tear on the body and interior, weather and paint damage, dents, issues with the electrical and HVAC systems due to age. Even if battery packs are $8000 or even $5000, folks will still be wondering whether it's worth it to replace them, much like engine replacements are reasonably uncommon in older gas-burning cars due to other wear and tear. At some point between 12 years and 20 years, 150k and 200k miles, the battery pack will actually die, and that's pretty close to the average lives of gas burning cars. That's *without* the gigafactory.
I'm elated that Tesla is working on producing cheaper, higher-capacity, more performant batteries, to bring the costs down on their cars and make the EV experience better. But longevity? It'll be nice for sure, and I certainly look forward to cheaper and longer-life EV batteries, but things are good enough today that I'm just not sure that's the biggest issue Tesla has to deal with. There's a lot of FUD in the non-EV community about EV batteries, when in fact, long term reliability on Tesla's existing packs has been quite good.