The industry consensus is that cells reach EOL when they only operate at 80% of their designed capacity.
No body knows what Tesla will do in 3, 5, 8, or 10 years. The battery packs may be covered by warranty, some sort of insurance, or just the owners. These are expensive cars, and unless they depreciate like crazy, they will have affluent owners. Maybe tesla will offer financing? idk
They could also use new, cheaper, better battery technology in a new "sled" so its not just new batteries, but newer better cheaper batteries.