It's a shame that RPi didn't just adopt a proper PD interface for power. For that matter, if they had USB-C + TB/USB4 with display support, then I could just plug it into my display without any other cables like I do my laptop, with all the peripherals connected to the display.

That would absolutely be a better solution but I meant in hindsight.

Any currently existing (to say nothing of two years ago) "TB/USB4" chipset would dramatically increase the price of something with a retail price on the order of $50.

With that said, DisplayPort Alternate Mode would be considerably more straightforward.

Apparently the RPi 5's SoC already supports USB-C display alt-mode.. unfortunately they don't to proper PD negotiation, which would not be considerably more expensive. There are cheap vape pens that support PD negotiation properly.

Are you sure these "cheap vape pens" don't just use 5V3A, which doesn't require any PD negotiation at all? (a lot of them screw even that much up, and a lot of people confuse "PD negotiation" with simply having the right resistors on the CC pins)

There is real cost savings here -- the RPi5 avoided the need for a buck circuit, and for that matter probably a dedicated PD controller chip.

In contrast, in the context of a "cheap vape pen" you have a battery which means you need to be able to convert to (and from!) battery voltage, so you need that conversion circuitry anyway.