You cant rely on your employees. What every system you use you will still need an IT contractor 'on call'. If 10% of your staff cant work one day because their computers have caught a cold what are you going to do? With a decent IT firm on contract you should get most of them back in a few hours. Also the IT firm is a large part of your companies disaster management plan, Can your business run without your PC's - no server no PC's? It only takes a rampaging digger driver a few seconds to remove your internet connection or power cable and the whole lot goes - at least with an IT firm at your beck and call they can set you up to run from home or anywhere else for a time or use another offices server.
Even with a cloud based system you will need IT support, in fact it is more essential to keep track of the 'who can do what' on the system.