A Mac's RAM usage

Am I right in thinking that the latest Macs, as they are, are designed to use as much as available RAM as possible, so as to optimise performance?

Correct. Idle RAM is of no use, you want instantly-available RAM, and that is what Mac OS X is now optimized to provide.

In Activity Monitor, check the memory pane to see the memory Pressure graph. Green is good.

The amount listed as Cache is copies of previously-read data from disk.It is released instantly if needed when an Application makes a request for more RAM. Some RAM may be compressed to save space, and it can be uncompressed and accessed much faster than a disk read.

today's Macs use a lot more RAM doing the same things they used to do with a lot less RAM?

I think your statement is partially correct. But todays Macs do it in a flashier way, with more flourishes, and often more additional features available.

Some of that is just "eye candy" (like translucency), and if you do not use the additional features, there is no additional value provided (e.g., "displays have separate spaces" if you never use it).

But years ago we had no realistic expectation that we could edit movies in essentially real-time.

The MacBook Pro Retina with soldered memory is a special case.

Apple has discovered that soldered RAM is vastly more reliable that DIMMs in sockets, especially with a portable, battery powered computer.

They also found that at huge-volume wholesale prices they pay for RAM chips, there was not a lot of difference to include MUCH more RAM, so that you would not likely be wanting for RAM over the expected life of the product. So 16GB is an option, and it is all soldered to the board with no mechanism for upgrade.