Whether or not they realise it, I think people are building brand new systems right now, using cutting-edge technology, which will survive until 2045.

Some arduinos with an ethernet shield and a super simple web backend?

Nah, somebody would say it need json-serialized config files, a flash-file system, a rails-based admin login running on a few amazon instances for fail over....

"Simple" Ha. There's this whole coprocessor in the ethernet shield with its own proprietary implementation of TCP/IP and who knows when someone will find a serious bug in it. Then in 30 years you have a TCP/IP stack with a serious security issue and no way to patch it!

Ethernet is great, but thirty years from now it and/or RJ-45 might sound just as antiquated.

Doubt it; 10Base-T is 26 years old, and still works with today's modern 10Gbase-T NICs.

There will be backwards compatibility options for an awfully long time.

How right you are. On the other hand I'm continually surprised that my paycheck in 2015 comes (more or less) from a descendent of an operating system I learned in the 80's.

Sure. And then it's time to retire it, after a job well done.