1. Say your 300 mile battery loses 30% in 5-10 years. So now you're down to about 200. In the winter in some areas you might lose another 30 to 50%. So now you're conservatively down to 100 miles. This still can be enough, but it's low enough where it's a hinderance.
2. The equivalent smaller battery as you've stated would drain faster, but it would also be substantially cheaper to replace to begin with. As a result the cost of the battery relative to the car is cheaper and the economics if buying a new battery makes more sense.
Not to mention that only Tesla's have the super charger network, so it makes even less sense for non-Tesla EV owners.
A battery range longer than the average trip distance * 2 is unnecessary. Ideally you'd have "just enough" + infrastructure to charge at said locations + gas to go further if necessary.