Since when do petrol and Diesel cars last more than a Million miles. Modern Batteries will last a lot longer than 150,000 miles.
Like any battery it will degrade overtime. Manufacturers battery warranties, guaranteeing an acceptable battery capacity of at least 70% of the original specification after seven or eight years. Like an ICE vehicle the bettery you look after it the better – a battery that has spent its time being trickle charged will degrade less than one that has spent its charging life on fast chargers. Also some battey chemistries are better than others.
Yes its credible a diesel car is capable of a million miles if looked after however its less epensive to repair an ICE engine out of warranty, than it would a battery.