I can understand that reasoning - I don't want to steer this topic in the direction of "defend that thing other people said they don't like" but, well, I'll do that. I think what makes their childhood friendship work for me is that I feel like all friendships at that age are extremely shallow, but that doesn't stop them from being extremely strong either. At a young age, you can basically find your best friend when you learn you both really like Power Rangers and from then on you guys are ride or die together. Every time they would flashback to their childhood, I was in the mindset of "this is just fun, innocent childhood friendships - nothing they're doing right now is especially deep but they're kids so everything they do is the most important thing in the world to them." So then when something extremely traumatic happens that interrupts that simple life, everything surrounding that event becomes that much more important to them.
In that sense, I feel like it's very relevant to people who have dealt with grief and trauma. For example, if you were a kid who was kind of a brat to your parents - you would whine and yell every time things didn't go your way - that's not necessarily a very deep or complex relationship and you're very likely to grow out of that phase eventually. But, let's say one of your tantrums had your mom get exasperated and step out of the house for fresh air, then she gets run over by a car. Suddenly, you might start reevaluating your childhood behavior as something much deeper than it actually was, because this otherwise unexceptional aspect of childhood resulted in completely disproportional consequences.
So that's the part of the childhood friendship in Anohana that really affected me. It was just kids being kids, but through some cruelty of random chance, normal childhood activity resulted in something horrible.
