There are very few kids who end up in this situation under medical advice compared to the extent of the problem.
I've had the same from the food angle. My eldest is 14 and was an utter horror show when it came to food - properly phobic about a lot of things, food of any kind was massively stressful, would starve himself rather than eat an unsafe food. For a couple of years there were about five foods he would eat and the dietician was very clear that calories were the priority for a long time. The rejoicing when he tried a fucking chip was ridiculous.
It's taken a huge amount of work to get to a point where he has a more or less decent diet, under medical supervision. The kind of diet set out in the OP would have been a pipe dream for us for years and years.
But general health advice should be aimed at the majority, not the rare outliers like you and my son.
The majority of kids don't need sweeteners to accept milk and water from the outset, any more than they need a diet that is based around bread, yoghurt and peas.
(I happily feed both my kids the dreaded cereal, processed food more than I probably should, and sweet drinks on occasion. At school DS1 consists entirely on sugary junk. But where parents can, setting their kids up with good habits when they're little can save a lot of problems down the road - I've never got over the number of kids who were in hospital to have multiple teeth out under GA when we were in for a different surgery.)