This kind of reasoning is perfect if you want to halt all future innovation.

Why would someone advance the state of the art in any device if the immediate result is attacks for the new device being a "monopoly" within the scope of the new device or improvements?

The legal term is abuse of a dominant market position, which does not only apply to pure monopolies ...

What? Aren't AMD, ARM, and Intel advancing the state of the art of CPUs because they have open platforms, and have no monopoly or gatekeeper position on what software can run on their platform?

I can't see why you would be unable to advance the state of the art if you don't block (thus allow) everyone from building on your platform.

What about fridges, toasters, smart TVs, playstations, watches, etc.

All devices made with purpose in-mind and only allow the manufacturer's firmware to be installed and are not "open platforms". In the case of smart TVs and many game consoles, you have a direct comparison to phones as they have a single app store with no competitors allowed on their platform.

Should all of these single purpose decives have to bare the burden and cost of opening up their platform for competitors too?

In one sentence you say "direct comparison," and in the next you contradict yourself by using the words "single purpose devices." A phone is a general computing device. A smart refrigerator is not. Is there even an app store on a smart fridge which contains third party apps?