My youngest was very adamant that the training wheels were staying on for a very long time or he would not get on the bike. My husband decided to test this and threw the training wheels away. We promised to hold onto the bike and help him balance, and the kid refused to even try. After 8 months of him not touching his bike, I bought another set of training wheels. I think we made the wrong decision - he had LOVED riding his bike before, and now, even though it's been a few years and he finally asked to take off the training wheels, he doesn't ride nearly as much as he used to.
With the training wheels, he had to learn to be careful not to steer into the grass, and to make wide turns. That meant that quite often he had to get off the bike and push it back on the sidewalk. That was a disruption he was prepared to tolerate.
Eventually, he figured it out and requested the training wheels come off. I think this was the result of excessive teasing by his teenage stepbrothers.