I remember my Gentoo days freshman year in college. I spent more time compiling updates than actually using the computer.
I recall gcc3 -> 4. The prevailing "wisdom" was emerge --deep (etc) world ... twice! My laptop was left for around a week trundling through 1500 odd packages. I think I did system first, twice too. I left it running on a glass table in an unheated study, propped up to allow some better airflow.
One of the great things about Gentoo is that a completely fragged system doesn't faze you anymore. Screwed glibc? Never mind. Broken python? lol! Scrambled portage? Hold my beer.
I have a VM running in the attic that got a bit behind. OK it was around eight? years out of date. I ended up putting in a new portage tree under git and reverting it into the past and then winding it forwards after getting the thing up to date at that point in time. It took quite a while. I could have started again but it was fun to do as an exercise.
I still haven’t decided whether or not I should be embarrassed that I mainly bought a 16-core CPU to run Gentoo.
My current Gentoo system seems to have existed since 03/29/21, so roughly two years now. In the time period, the time spent compiling packages has accumulated to 5 days, and my CPU takes ~140W at max load (Ryzen 3900x).
If I did my math correctly, this comes out to roughly 16KWH accumulated energy across two years.
We can compare this to a gamer, who spends 1 hour per day gaming, for 2 years, on a system that takes 300w while running a game, and this comes out to 230KWH in total. That about 15x as much energy spent by a fairly lightweight gamer on a very average system.
It's also worth noting that the majority of packages build in under 1 minute on my system, the vast majority of compile time is spent on things like Firefox, Rust, GCC and a few more.
This is just a very silly thing to be concerned over, and if we are going to be offended at people for being wasteful there are much larger targets than someone building packages from source.
The possible combinations that Gentoo allows looks to me like a sort of Linux immune system in action. Quite a few "unpopular" flags will get used (lol USEd) somewhere by someone that will be more motivated on average to log a bug somewhere.
Gentoo also got the console shell look (colours, fonts etc) right way before any other distro. It's copied widely.
Wanting to have control over config via use flags for your system doesn't mean that there aren't packages were you don't really need that. Like if you only use Libre Office a couple times per year on your aging laptop, do you really care enough about the exact USE config to justify compiling it yourself? Even more so if you need it on short notice. Or if you only use Chromium/whatever to check that your website works with that browser but don't actually use it yourself, why bother compiling it.
IIRC there used to be a Gentoo fork (forgot the name) that extended this concept to all packages, so if you used default USE flags you did not need to compile things yourself.
[0] https://www.dropbox.com/s/w1zlftin1cojkhr/kernel_compile.mov...