I'm not saying that achieving good Wi-Fi reception for a device with more metal is impossible. I'm saying that achieving good Wi-Fi reception for a device with more metal is going to be quite difficult, but not impossible.
I've used to own a HTC 10, which has a completely metal body, and
has really poor Wi-Fi reception, despite having antennae lines on the back. (I think one reason is that
the Wi-Fi chip used on the HTC 10 is not that great.)
I think one of the reasons why the upper portion of the OLED model's back is plastic is because of the
Wi-Fi antennae placement.
And that reminds me. Another potential caveat of using more metal is that the hardware could become too hot to touch, especially if people play in TV mode for long periods of time, and if Nintendo's being really aggressive with the CPU, GPU, and RAM frequencies when in TV mode, since
metals are also very good conductors of heat.
I mention this, because I have a launch Nintendo Switch. And I've noticed that when I play in TV mode for 30+ minutes, my launch Nintendo Switch is very warm to the touch (still comfortable enough to touch without hurting myself, but fairly close to the point of being too hot to touch).
And introducing more metal could make the hardware more susceptible to being too hot to touch, especially as I've mentioned before, people play in TV mode for long periods of time, and if Nintendo's being really aggressive with the CPU, GPU, and RAM frequencies when in TV mode.
And because metals are very good conductors of heat,
metals are very good at dissipating heat, which is good for ensuring the SoC, etc., remains at a reasonable temperature when running, but that could be problematic if someone wants to remove the hardware from the dock 30+ minutes later.
Perhaps having the top portion be plastic could help with not only the Wi-Fi reception, but also allowing someone to remove the hardware from the dock after 30+ minutes.
Or perhaps Nintendo could install fans inside the dock.
Who knows?