Yeah, no complaints on RJ45 for me. A smaller connection would likely mean not being able to roll your own cables, which is an issue both for a lot of people and lots and lots of organizations. That plus the fact that you are rather limited in how small you can make it, the 23-25AWG wires of cat5-6a are rather larger than the typical wires in a USB cable (often 28-32AWG), so the connector is generally going to have to be a fair amount larger as a minimum requirement.
RJ45 isn't tiny, but it also isn't that huge either. My laptop has a nice compact port. You'll see a number of them around, the bottom hinges down on a spring so it makes the port roughly half the size when closed, which is only something like 10mm or so. My ultrabook ain't super duper thin, but it still well under an inch. I think something like .8in with lid.
Really, there is only so small and light you can make a laptop before it just makes no sense anymore and I think roughly the range we are talking now is it. They aren't too heavy for fatigue nor too thick to fit in a bag resonably, heavy enough to balance the weight of the screen or if using a touch screen, you aren't tipping it off your lap with a casual touch.
If you wanted to say tablets...well, hinged RJ-45 port like you see on some ultrabooks, or if you need thinner, USB to ethernet adapter (it is what I use on my T100, a USB3 to gigabit adapter. Works great, though I really only use it when my tablet is docked with the keyboard, but I have verified functionality when using a micro USB to USB adapter to plug it in).