That's a knee-jerk reaction that you probably already know has some serious problems ;)
People who are working 40 hours a week have no capacity to take on some important task with no notice unless they go past 40 hours a week. Which generally doesn't cost any extra money.
Hiring a contractor to work 40 hours per week is going to get really expensive really fast when you need them to suddenly ramp up beyond 40 hours on no notice. If you can even get them to do it!
If you're any kind of normal modern business, this is going to eat all your cost savings and probably even cause you to unnecessarily miss a lot of deadlines.