Well, if you want to look at nursing simply in medical terms, the longer you nurse the better. Various sources say the human immune system is not mature until about age 5 or 6.
The World Health Organization (WHO) and UNICEF, and
the American Academy of Family Physicians say for optimal child health, children should nurse AT LEAST two years.
But you are asking a deeper question. You are asking is it unfair to teach a child you will always be there for them, when real life gets in the way, when the cold cruel world is not fair, etc. etc.
Anyone with more than one child knows you can never give ALL the children 100% of what they need. This is a painful and humbling fact. I still believe (as I've posted at MDC before) that is is our Western, live-in-separate-houses, nuclear family life that is to blame. For the millenia of human history that AP has been practiced, humans lived in groups. When one mom was cooking or busy or exhausted, their friend/aunt/cousin/mother-in-law would step in and help. The INTENSE, EXHAUSTING, PHYSICAL work of childrearing and cooking (as well as caring for ill adults) was SHARED.
Not that I want to move in with my crying-it-out, early weaning relatives...I am set in my ways and I am not ready to give up my space and my privacy...I'm just saying our current way is difficult and not the way humans were designed, really.
It's hard when we are torn in so many directions as mothers, but nursing is such a valuable tool for comforting toddlers (not to mention the health aspects.) And a two year old would still need you just as much if you weren't nursing! But they are also adaptable and the firm foundation of love you have built by AP-ing will help both of you survive any hard times like this. I have found that children are remarkable forgiving little people. Again it is humbling.