How were these early PCs any more closed than the IBM PC clones that followed? Some shipped with schematics and ROM listings. Smartphones have now been around longer than the entire pre-IBM PC era with no obvious movement towards any kind of opening outside of hobbyist devices. Game consoles have been around for decades without any such shift either.
They were more open than an iPhone, but they were not particularly open to anyone with less than a CE/EE level of computer and electronics expertise. They certainly were not designed to be modular and open like the clone PCs that came later.
I don't think that's accurate, just google up an Apple ][. But even if it were true, it wouldn't be strong evidence for your 'consumer hardware platforms become more open' theory given the reality of the longer-lasting smartphone era we live in now. Both the theory and the example used to support it seem starkly at odds with that reality, to me.
Game devices and smartphones were carefully designed to remain closed. Although the Pinephone and Librem 5 look like smartphones, and use many of the same components, they are far closer to phone-sized PCs. And so they're open from the start.
In addition to that concern, I would add that we've seen some of the opposite move happen - PCs change to be more like phones. The closed source OSes are all moving towards fenced off app marketplaces, with heavy sandboxing between apps. Even Ubuntu seems interested in this kind of move, with Snaps. We've also seen attempts to close the bootloader to "tampering", a restriction in place since the beginning of smartphones. We've also seen components like memory and batteries soldered into the computer instead of being replaceable. And so on.
I'm worried mobile phones are reducing the expectation that the average consumer has of being able to modify, upgrade, and repair their devices, and computer manufacturers are poised to take advantage of this, rather than the other way around.
