I’d rather have a miniPC-sized Raspberry Pi computer like an Intel NUC or Mac Mini* mounted on the back of a monitor instead of an all-in-one. Monitors have a much longer useful life so all-in-ones (e.g. iMac) eventually become paperweights or e-waste.
If they go the all-in-one route they should include an external HDMI port and a switch to choose between it and the internal Raspberry Pi.
Exactly, this is what the VESA mount on the back of your monitor is for. For an almost no effort solution, you can zip tie a pi to the mount bracket and screw it in (some monitors may even be able to power it via usb). Integrating into the monitor's electronics makes no sense.
This is pretty much how I had things setup but it isn't ideal as getting to the Pi is a pain and it doesn't work well with any hats so it didn't last long.
My thinking for a Raspberry Pi monitor would be to have the ports in an easier to access location on the rear of the monitor so hats, etc. can be made to a standard design.
Of course the monitor will likely out last the Pi inside it but there is nothing to stop it being used as just a monitor one the Pi instead is no longer useful.
If the stand/arm is using the actual VESA mounting holes, there are brackets that can hold a NUC either to the side or sandwiched between the stand/arm and the monitor.
If they go the all-in-one route they should include an external HDMI port and a switch to choose between it and the internal Raspberry Pi.
*: But upgradeable and repairable.