First, I'm not suggesting anyone tell mothers they have to nurse for any duration of time. I'm just suggesting doctors tell them that 2.5 years seems to be the minimum natural age of weaning/the minimum duration the child has been designed to expect. And further, that some moms choose to allow baby to wean when ready - up to 7 or 8 years of age when their "milk teeth" fall out! Second, if you know that a certain length of time is normal for a particular activity, then you just accept it -- it doesn't seem daunting.
No one thinks it is too daunting to tell women they must be pregnant for 9 months in order to have a healthy baby. We all know that human pregnancies last 9 months, and if you want a healthy baby, you have to be pregnant for 9 months. And while some babies survive who don't stay in the womb for the whole 9 months, they often require special medical care and many end up with permanent disabilities to varying degrees. Few, if any, women, would risk the potential damage to their baby and insist on an induction at 6 months of pregnancy because they found it too daunting to be pregnant for the whole 9 months, and were only willing to be pregnant for 6 months.