Because it's meant for ultra-slim devices that will be docked on Thunderbolt 99% of the time and that foldable RJ-45 jack is for the "in case of emergency break glass" scenarios, that 1% of the time when you need to patch into a server physically without wasting time looking for a dongle, not for you to constantly plug/unplug ethernet cables in your laptop.

If your uses case requires you to constantly plug/unplug ethernet cables in your laptop then you need a workstation class laptop with a full sized RJ-45 jack, not a sleek thin and light.

That one yes. But Lenovo has great collabsible Ethernet ports that are very durable and this laptop is more than thick enough to house one.

One careless snag of the cable would have to rip the aluminum apart I would think

The connector's on the top, the bottom is simply a retention clip. If it breaks, you can replace it with tape, or simply resting the laptop on a surface while the cable is plugged in.