1. A battery isn't completely dead when it reaches its cycle life. It's often defined as the point where the capacity degrades to 80% of its initial rated capaity. For an EV with a large battery, most of them will be able to continue to use the battery for a good while after that. A Model S 90D is still a very usable car for many people with even 50% range.
2. You assume that every EV owner will use many cycles. That's generally not the case. I use at most 2-3 gentle cycles (charging to 80%, never discharging below 20%) in a month on our EV. I have plenty of spare cycles in the lifetime of the car.
3. Time degrades batteries as well. For EVs with low mileage, you could be throwing away useful battery cycles by not using it for V2G.
4. Most batteries, by far, will not have to be replaced until the vehicle is over 10 years old. In 10 years, it might be far cheaper to replace EV batteries. Batteries will be cheaper and you'll have better economies of scale for replacements. You might get a better battery with longer range than when the car was new too.
I seriously doubt that V2G will cause a lot of people to have to replace their EV battery in the lifetime of the car. People who drive their cars the most will not use V2G because they won't have spare battery capacity. It might trigger one battery change, but then an EV might need a battery change at one point in its lifetime anyway.. and so it doesn't mattery unless V2G triggers the need for a third replacement. (In 10 years batteries might have longer cycle life too).
I seriously doubt that V2G will cause a lot of people to have to replace their EV battery in the lifetime of the car. People who drive their cars the most will not use V2G because they won't have spare battery capacity. It might trigger one battery change, but then an EV might need a battery change at one point in its lifetime anyway.. and so it doesn't mattery unless V2G triggers the need for a third replacement. (In 10 years batteries might have longer cycle life too).