Hardware

what is the max speed we can put through and Ethernet connection??

Copper-based Ethernet and DSL share quite a few similarities. So why can't you just keep easily increasing DSL bandwidth? Because to send more information you either have to send it at a faster clock rate, use fancier encoding (e.g. instead of just on/off you could also carry information using light polarisation or colour, if we use light of course) or literally increase the bandwidth and use more frequencies.

A larger bandwidth means using more frequencies, and higher frequencies drop off more quickly with distance, to say nothing about cross talk and other effects. You also get cross talk, interference, and other effects which raises the noise floor and makes it harder to tell a genuine signal from background noise.

Frequencies don't propagate at the same speed either, which also poses limits on the clock speed, because over a small enough time scale the signals blur out. You're also capped by hardware limitations.

Encoding more information not only requires more processing (see above), but you need to be able to hear what's being said. Same principle as talking in a quiet room, vs. yelling and having to repeat yourself a lot very slowly in a crowded area (fall back to slower speeds, retransmission of data, increasing redundancy).

You can add more pins and wires as you say, but then that means you need more wires, more shielding, it can make the cables harder to work.

I would estimate 40GBASE-T is probably a reasonable limit for copper-based Ethernet. If you switch to fibre that limit keeps going up and up, for now. And given the ability to multiplex different wavelengths of light over the same fibre, while I'm doubtful that's going to be needed in consumer grade equipment for a long time, that gives Tb/s of capacity using today's technology over a single fibre. And even single wavelengths are doing around 400Gb/s.