The durability of a commercial truck and affordable replacement costs
First of all, the entire car was designed for durability, like it would be if it was a commercial vehicle:
"Model 3 drive unit & body is designed like a commercial truck for a million mile life."
The battery modules alone would be good for about 1,500 charging cycles, which translates to 300,000-500,000 miles (480,000-800,000 km), depending on battery versions (Standard Range - Long Range).
" Current battery modules should last 300k to 500k miles (1500 cycles). Replacing modules (not pack) will only cost $5k to $7k."
If we multiply the expected EPA range of 325 miles (in the case of latest Model 3 Long Range RWD) by 1,500 we would get 487,500, which is close to the 500,000 miles. It suggests that we are talking about 1,500 100% Deep-of-Discharge (DOD) cycles.
The battery capacity fade for those who drive a lot could cause the necessity to replace some of the battery modules (four) instead of the entire pack. The expected cost is $5,000-$7,000 (for all the modules but not the pack itself, we assume), probably also depending on the type (Standard Range or Long Range).