I agree with your point in that children's birthday parties are very extravagant compared to when I was a child and it was basically family, cake and presents. Young children are allowed to be disappointed but I do wonder if much of the disappointment mentioned on SM actually belongs to the parent.
I think this is more to do with MN demographic than anything else. Elaborate birthday parties for certain demographics were definitely a thing in the Victorian and Edwardian ages.
On the other hand, they are less of a thing around where I live. Church hall and party games are about as far as we stretch. Or possibly a few select friends taken to McDonalds.
And for the record, church hall and McDonalds are favoured by the poorer families, the ones whose living rooms are so tiny you couldn't even fit in a dining table and where a garden is either non-existent or a couple of metres square of concrete. (You could go on inviting 6 children to McDonalds once a year for a loooong time before that added up to the mortgage for a larger house).
Dc have only ever had at-home parties, no one ever complained. But then we were among the more affluent of their circle and lived in a 3 bed semi with a garden large enough for 6 children to sit on the lawn at the same time. Not all our friends did.
Can't remember anyone ever complaining of being bored- except if they were in trouble for some different reason. Ime of children, "bored" might cover a multitude of concepts, including feeling bullied, feeling shy, having been told off, just having done something they shouldn't and being scared of being found out etc etc.