Yes, the US is highly dependent on the single E-7 at present. It has a much better RADAR than the old Sentry as well, especially for picking out low flying missiles against sea clutter. Obviously it can't be in the air at all times, but I understand from OSINT that many of the Shaheds are launched at night.

The ground location of the Wedgetail is obviously something the Iranians would love to know, in which case they would saturate it will ballistic missiles and it would be a race to get it in the air. I expect it moves regularly but that would create significant logistical issues too. Unlike Patriot batteries, which can be heavily sandbagged and routinely shuffled between prepared positions, a 737 on the tarmac is incredibly vulnerable.