A typical car battery is rated for thousands of cycles. 3000 cycles is a number you hear often. If you have an EV that has a range of 300 miles, we're talking about 900000 miles. That's more miles than the vast majority of cars will drive over their lifespan. The average driver drives less than 15000 miles per year. The car will die before the battery typically, not the other way around. And the battery isn't completely dead at that point, it should still be good for 80% of its original charge. The vast majority of EVs ever produced are still driving with their original batteries and will be for years to come.
A second point is that EV batteries are designed to discharge in a few hours to drive the car. There is no way you'd discharge at that speed via v2g. Most home connections charge/discharge only at a few kw. It's a comparatively light load for the battery and the wear and tear is pretty modest. Battery degradation happens a lot faster if you stress the battery. There are some indications that suggest that v2g discharging has only a very minimal impact on battery life. You wouldn't fully discharge the battery. Not even close.
A third point is that with smart chargers, you would be able to control when the battery is allowed to discharge and how much. A million cars contributing just a single kwh each is 1 gwh. That's a huge amount of power. You would not fully cycle the battery typically but maybe only a few percent. If you start with 3000 cycles and you use 10% of that every day (which would be a lot), you could do that for 30000 days. That's longer than most humans live.
Finally, instead of providing power to the grid, you could use a system like this to power your house and make save significant amounts of money in the evening by using your car instead of expensive grid rates using power that you got from your solar panels during the day for free.
Those savings can really add up over the years. If it worries you, you can invest in a house battery instead. In the end the argument has to be financial and v2g is probably cheaper overall than that. People are spending tens of thousands of dollars to put solar on their roofs. So, throwing their car battery in the mix is a comparatively small step and a perfectly rational thing to consider. But the numbers have to make sense for people to opt in to that.