It breaks the internet because it breaks IP - the internet protocol. Segregating devices onto separate networks that are not routable to each other is something that reduces the overall functionality of the internet, not enhances it.

It doesn't help that it's a shitty hack which causes endless lost time either spent wondering why something is broken, or through designing entire protocols just to get around it.

If you consider that "breaking" the internet then many thing break it. That seems like a weird argument to make.

I understand where you're coming from with that – but I can imagine a different Internet, where devices where by default globally routable but firewalled. That could even have affected the current trend towards 'stuff it all in my proprietary cloud system'.