I'm commenting sentiment of outside an enthusiasts forum to which you agreed under the premise that regular people don't understand this. If they don't understand this, then they likely aren't even asking for it.
I have explained and given examples why people not thinking about this in terms of closed air open ecosystems and them having opinions on elements of both aren't the same thing.
I have also explained why I don't think that people buying Apple products cannot be read as unequivocal support for closed ecosystems, like it's often interpreted here.
Furthermore, reasons why people buy something can be multiple and someone who likes open ecosystems may still buy an iPhone because they like the hardware. They may even start using non-Apple services if there was more competition. Maybe not.
If you want to reduce all of people's motivation down to their purchasing decision that's your prerogative, but I don't think it's true and it would make for some bad public policy making, least of all because I'd argue that in terms of public policy consumer preference is not the only thing you'd have to consider.
I'm not convinced the majority are asking for it to be an open system. Therefore the law shouldn't be forced unless we have hard data showing otherwise.
Lots of people clearly don't care about copyright laws at all, that doesn't mean we should abolish them.
Arbitrary distinction just like how EU defines gatekeeper status. It's purely subjective to set a threshold of X users or sell X number of devices that you're suddenly treated differently.
It seems like these rules aren't based on principle but based on being against big companies, even if big companies are doing the correct things.
If by arbitrary you mean set by the legislature according to some considerations that a group of humans decided where the right ones, then yes it is about as arbitrary as any law because that's how laws work. It is quite common that someone will decide at which threshold obligations, duties and right apply and whether they will be the same or differ depending on some criteria.
Generally that's a good thing because you wouldn't want to treat your trillion dollar multinational based on the same principles because both are companies.
Whether the DMA sets the right thresholds is obviously a different question, but I find it fairly easy to justify why we should regulate dominating mobile phone platforms -- really there's two and both are now captured -- that more or less everyone uses, to varying degrees, for participation in modern society and video game consoles which a minority of people use to play video games differently.