I would hardly classify ethernet as "necessary"; wifi serves the same purpose in most situations
Yes. Exactly. "Most". For the odd time you don't have it though it really sucks. The hotel with only wired in the room. Or the day your wifi router craps on you and you need to wire in to fix it. Or the office that has wired everywhere because ethernet just works, whereas wifi has all sorts of issues... signal strength, bandwidth contention, more prone to random disconnects, etc.
An RJ45 ethernet port adds like a nickle to the cost of a laptop. And if it has to be a couple mm thicker so be it. Fill the space with some more battery.
if a vendor wants to offer an ultraportable without one that's fine. But any vendor that removes ethernet from their entire laptop lineup from ultraportable to pro-desktop-replacment has its head firmly stuck in its ass.
With the rollout of USB 3.1 and the USB-C connector, we are going to start seeing USB only ultraportable laptops. USB-C adapters will be used for power, video, ethernet, dock connections, etc. The days of separate dedicated physical connectors will go away, similar to the way that parallel and serial ports were replaced by USB. My prediction is that we will see this within the next 3 years.