It takes 10-15 years to become a doctor.
It takes 2 years to decide what college you want to go to.
10 years is too long to decide if you want to open yourself up to financial rape and spend the rest of your life with one person? Really?
Terrible analogies.
It takes 10-15 years to become a doctor because training is intense and long. The actual
decision to become one was made long before that. The length of the journey is known before making that commitment.
Being engaged indefinitely with no actual idea of when it's going to happen is a silly concept. You have made no actual decision. There is no long road to getting married like there is to be a doctor. All you have to do is go to a courthouse, fill out an application for a license, pay a fee.
If you decide to get engaged, that would imply that you want to get married. If you're scared about marriage or don't want to get married, why not get in a relationship with someone who doesn't want to bother with it? Those people are out there.