DontEvenThink, I am really confused about the way you are interpreting the WHO guidelines. I don't think they are vague at all.
Exclusively breastfeeding up to 6 months= nothing but breastmilk until a baby is 6 months old
Continue alongside complementary food up to the age of 2 years= breastmilk and solids until a child is 2 years old
and beyond= longer if you fancy it
This is the meaning taken by HCPs and breastfeeding organisations across the world, I'm not sure why you're getting a different meaning from it. The WHO wouldn't use the phrase 'up to' to mean two different things in one sentence and they wouldn't set a limit of two years and then say you could go beyond it if you like. The 6 months and two years recommendations are NOT limits. They are targets.
This is from the book Breastfeeding Answers Made Simple which is used by HCPs and breastfeeding counsellors-
"Research has found that children weaned between 16 and 36 months have more types of illness, longer duration of illness, and require more medical care than breastfeeding children the same age (Gulick, 1986). Also, weaned children between 12 and 36 months were 3.5 times more likely to die than those still breastfeeding (Molbak, 1994). No matter where a child lives, weaning before age 2 is associated with greater incidence of illness and death."
If you want to look up those references, they are-
Gulick, E (1986) The effects of breastfeeding on toddler health Pediatric Nursing, 12, 51-54
Molbak, K (1994) Prolonged breastfeeding, diarrhoeal disease, and survival of children, BMJ, 308, 1403-1406
I'm sure the risk of death in developed countries is tiny, but improved infant health is the reason for the WHO's recommendations.