Reading through this thread is certainly interesting to me.
When I started here in 2012 or whenever it was with a 1.8ghz single G5, that computer was a workable back-up for my 2011 MBP. There were some work-arounds to get things done, and it wasn't too long before I ended up with a Quad, but it was inexpensive and filled my need for a desktop. At the time, I'd sometimes see Mac Pro 1,1s for then-current MBA prices, and think they were a bargain(MP 1,1s from what I remember started really going down around the time Yosemite came out since Lion was rolling off security updates and it wasn't widely known then that they could be upgraded past Lion).
Back in those days, aside from running Office 2008, Dropbox worked, Youtube was very easy especially on a G5, and a bunch of other things just were still natively supported. You could ALMOST get by then with Safari and Firefox in Leopard, although Tenfourfox was around and at the time had feature parity with the then current Firefox, and Leopard Webkit was very fast and capable.
As I remember, 2015 was about the time stuff REALLY started breaking. I continued to use Dropbox for a while(something fairly important for a lot of uses for me) by spoofing 10.6 though a couple of terminal commands, signing in, and then changing back to 10.5 since the desktop client could still communicate fine but just wouldn't sign in as long as it saw you were running 10.5. There were a few others that fixes popped up quickly as things quit working, but after a while easy fixes became-to-me-tedious work-arounds and more proof-of-concept than things I could just rely on to work(the Dropbox thing was something I'd only have to do on restart, and it would be fine).
Heck, even in 2015 when MR was still on vBulletin, I could browse the site pretty easily(not quickly, but more than tolerably) with my 9600/200MP running OS 9.2.2 and Classilla.
As more and more stuff broke, it became less and less practical to use PPC computers as a primary computer. Several of mine were relegated to specialty roles-I used a dual 2.7 G5 as a scanning workstation up until I got my MP 5,1 in I think 2018(and I'd only been using it for a year or so, replacing a dual 1ghz Quicksilver), but it wasn't easy to navigate the web and do other things, and it increasingly became "sandboxed." I enjoy seeing the efforts to make things work in 2021 and would really enjoy working on them myself if I had the skills, but more and more the work arounds get complicated and seemingly break after a short stint of use.
I've just recently upgraded my 2012 MBP to an M1 MBP. I really, really like the M1, but I'm constantly regretting not getting 16gb RAM(the reason why I didn't is complicated). I can get by doing lighter work on 8gb in an older Intel computer, but as an example doing a Lightroom import+DNG conversion while trying to browse the web even on the M1 is miserable with 8gb.
On the other extreme, I never went to 16gb in a Quad, but did get one up to 12gb. I have systems where I have quite literally more memory than I can make use of but did it just because, including the 88gb in my 5,1(I need to fix that to 96gb at 6x16 rather than my current 5x16 and 2x8gb-I bought 6 16gb sticks, one was bad, and the seller went ballistic, refunded me, and blocked me from buying claiming that it was my own stupid fault for ruining it because it was "Server RAM" and using it in a desktop "will fry it" since I guess the seller didn't understand that workstation class computers are a thing). Another ludicrous one is my 9600 with 1.5gb, again more than I can possibly use.
Still, though, I've never tried to take a Quad to 16gb. It's fun bragging rights, and I'm not against those by any means, but it's just never been a high priority for me. It's incredibly difficult to use even 12gb at least in my useage. The big issue is that you can probably count the number of PPC-native 64gb programs on one hand. The heavy hitters like Photoshop(the stereotypical resource heavy program) never went there, and I really suspect it was because PowerBooks were still G4/32 bit and I suspect the big software developers didn't want to shut off the mobile market at a time when laptops were getting good enough to be even a moderately heavy user's only computer rather than a supplement to a desktop. That means that a lot of the "big" software is stuck at 3gb RAM whether or not it would benefit from more. So, there again, outside heavy multitasking with that sort of software, it's hard to even use 12gb.