It's important to consider the sometimes positive externalities of the current levels of defense spending. Even more so, it's important to consider where we or others have come to rely on those positive externalities.
For example, I'd be perfectly happy with the US withdrawing from many parts of the world and closing foreign bases. However, I do imagine there would be sometimes not so subtle consequences to doing so. Is the current balance of global power sufficient to prevent large scale and disastrous conflict? Do we risk tipping that balancing in one direction at some point?