1. People usually start a side project because they are unhappy at work. If they are unhappy, they are unlikely already giving their best
2. On another note, a side project often gives your a great deal of satisfaction, accomplishment, control and responsibilities that you wouldn't get in your daily job. This balances your morale and will make you perform better both at your daily job and at your side project
3. I worked on a number of side projects in the past. Some successful, some not. But in ALL of them the learning was priceless. I didn't start side projects because I wanted to start a business and go all-in. I did start them because I wanted to learn things that spaced beyond my daily tasks at work. And it worked brilliantly.
Also, if you've a mortgage to pay and a family to maintain, you have some responsibilities that make the "go all in" move quite a big risk. And I do believe it is possible to start a business starting as a side project first - it just takes much more time and energy and, most of all, patience.