I think OSs in general just eat a portion of whatever memory you give them. Right now I'm puttering around with a dozen tabs in firefox, in fact my biggest memory hogs right now are firefox with 3.5gb and apple mail with ~500mb, not really doing anything else, and somehow 12GB/16GB are in use. Better for the ux to keep things open in memory if you have it to spare, I suppose.

When you are memory constrained, you can definitely tell. Everything comes to a halt and you just twiddle your thumbs between commands. This 16GB machine I have shipped with 4GB which was painful even 8 years ago when it was released, and I upgraded myself to 8GB 6 months into ownership. A few years later when javascript became more pervasive on the web, I hit memory constraint on 8GB a lot just from having tabs open in chrome, back when it was perhaps more of a memory hog, so I opted for 16gb and haven't had issues since.

I think at 16gb you should be set for at least 5 years. Most people, even a lot of devs on company issued equipment, are working with 8gb complaining about it right here in this very thread.

If you have larger requirements, a lightweight, thin laptop with a teensy fan isn't for you. Even if it had the hardware specs, the physics of heat dissipation don't work for you and you are better off spending the same money for more hardware sitting in a box under your desk. Me and my sore back are eyeing this up, all my computing is done on a cluster anyway.