If you were going to use it on a low wattage system you might be happy using a fixed voltage fan swap at 5v but most wouldn't want to do that as it would lower the usable rating of the PSU.
I am a proponent of the fixed voltage fan swap, if only for its simplicity (anything too involved and I'd rather just buy a new part). You are assuming a lot to say this will lower the usable rating of the PSU (ie that the users ambient conditions will be the same as those used when selected the fan control and that there was no safety margin inherent in the design). I think it is far more likely that you will reduce the usable lifespan than the available power. Look at the way SPCR torture tests PSU, yet very few go into emergency shutdown and or fail to deliver their rated wattage.
There is a LOT of assumptions in that statement. Looking back at it I should have said some
instead of most. I was assuming that a SPCR regular that does a fan swap to a fixed 5v PSU fan is going to fix that fan at a very low RPM. I'm also imagining the worst case scenario when throwing out that warning. I've definitely seen users on SPCR talking about a fan swap where they bought the slowest fan they could and then fixed it at 5v or put it on a fanmate and turned it just above stall speed.
Noctua NF-s12-1200 at 5v 640 RPM
Noctua NF-s12-1200 at 5v 390 RPM
Nexus D12SL-12 at 5v 490 RPM
Scythe S-Flex SFF21F at 5v 550 RPM
Scythe S-Flex SFF21E at 5v 400 RPM
SCYTHE SLIP STREAM SY1225SL12M at 5v 720 RPM
SCYTHE SLIP STREAM SY1225SL12L at 5v 420 RPM
SCYTHE SLIP STREAM SY1225SL12SL at 5v 170 RPM
If you pick a more reasonable fan you might be OK up to hundreds of watts and really who needs more than that? I never break 200W on my systems even when gaming so if I took a 850W PSU and swap the fan such that it would shut off at 400W due to heat I'm still within a big range of safety margin and your are right I'm not doing anything but shorting the life of the PSU slightly when I run it at >100W.
But knowing the internet I toss the warning out there so some gamer doesn't fill his system with multiple video cards and a raid array then swap the PSU fan down to almost nothing and wonder why his PSU is less stable or dies of heat death sometime down the road. I often word things for the lurker that may read my post way down the road and act on it but not partake in the discussion.
Maybe I'm being too cautious but the thought is there in my writing sometimes.
This PSU doesn't give 5v to the fan until it's hit over 400W but at that voltage the stock fan was at 1370 RPM. I don't know how much headroom it has if you replace that fast fan with a slow or very slow fan.
Here is one that hits 5v between 300W and 400W and is probably about 1000 RPM at 5v. No big deal to the average user and they could swap for a slower fan.
5v at just over 200W for this one. It'll work with a slower fan but you are surely eating into the design margin. Your choice of fan will matter more on a low wattage PSU like this.