instead of the booster aspect. Pegasus lauches are done over water. This one will also launch over water. The reason is simple - range safety. If the launch fails, it's a lot safer for the debris to fall into the water instead of over land.

As far as the potential weapon's aspect, range safety is still a factor to consider. There are a lot of countries that wouldn't want us to launch over their land, partly because of legitimate concerns for safety and partly because they're not friendly to the US. There's no way I'd want to base such a system in Western Europe.

The range of the launch aircraft is inversely proportional to the launch payload. Every plane has a useful load, which is equal to the takeoff weight minus the empty weight. Longer range means more fuel, which must come at the expense of payload. Mid-air refueling can mitigate this somewhat, but that adds expense, complexity, and new failure modes. This current proposal has very small payload margins to start with (130KG to low inclination LEO, about 80 KG to a sun-synchronous orbit). Increasing the aircraft range very much would wipe out the ability to launch much of anything.