My only goal was to put some numbers to this topic. I did read those papers but wasn't especially interested in explaining every number.

Motherboard price

$30 is not a normal price that I know for a motherboard. That's like the clearance, discount price. And they are not manufactured in a country that charges our electricity rates - industrial rates in China likely don't even come close. I picked numbers closer to our electricity rates because that's what I thought you meant. I came across others numbers similar to the link I provided for kWh/kg but that was the simplest to link for the sake of not making my post way too long. As I said, I don't know how these papers and websites came up with relatively similar numbers but I went with it.

I'd consider $100 a more appropriate price point but this doesn't change much so that's cool.

Power Consumption

And the parent post was talking about a desktop computer. One that has a separate monitor(s), peripherals, and more power-hungry devices in the case. A desktop does not consume 40W. You called power consumption between a desktop and 3D printer a wash so I'm happy to continue with that assumption rather than bring it back up.

I figured using a laptop for 8 hr/day across 4 years (as per the paper) was pretty realistic. I fully intend to have my 3D printer for 4 years but very few have reached this amount of time for us to know if it's a realistic timescale for comparison. My 3D printer with heated bed uses less than 160W at steady-state but that's fine. Running your 3D printer for a couple hours a day makes the laptop power consumption a wash. But remember that a desktop uses a lot more power than the laptop and your average 3DP user doesn't get anywhere near 2 hours of use every single day from their machine.

Section summary: Desktop likely has a far, far higher environmental impact than your average 3D printer for power consumption if we try to adjust for normal daily usage of both devices.

Transport Footprint

Given that a 3D printer weighs more and is larger than a laptop

Again, parent comment was talking about a desktop. This means cost to transport all components, case, peripherals, monitor(s). Compared to the weight of some of the more popular 3D printers in this sub, I'm also calling this a wash.

Now continued transportation of 3D filament and cost to extrude are interesting factors to consider. And they certainly are real.

This site (complete with source) says that we use 0.5 kg CO2eq to ship 1 metric tonne of material 1km by air (using a 747). Way way less if by other shipping means (haha, Zeppelin). But let's go with it. That's the same as sending 1kg of filament 1000km. That works for me! But let's double it anyway to 1.0 kg CO2eq per kg of filament (again, working in favour of your argument).

Manufacturing Filament

Average carbon footprint as it relates to electrical energy usage in the US seems to come in around 0.40 kg CO2eq / kWh. If it takes 1 kWh to make 1 kg of filament (thinking industrial scale), let's use this number too. That brings total carbon footprint to likely under 5 kg CO2eq / kg produced and shipped by air. Not 3.2+15.

Computer control

Finally, lets be real here and realize that most folks running a 3D printer are controlling the print from their computer

How is this an argument? Do you dedicate a desktop computer to controlling your 3D printer and do nothing else with it the whole time you're using your 3D printer? Because that's the only way you can clump it in as part of the 3D printer system. Additional power consumption from SD card printing or running from an embedded Linux board is effectively zero and this sub advocates against tethered USB printing from your main computer. Those that use their main computer are likely doing other things with it as well making this comparison not good. You can have a complete 3D printing system running at 160W and average desktop power consumption is ~100-150W according to a lot of the internet. Let's keep things simple, shall we?

Bringing it home

With power as a wash, a 1kg spool of filament produced and shipped was liberally put at around 5.0 kg CO2eq / kg. Your average user might run through what, a spool a month?

This source shows that a desktop has a life-cycle carbon footprint of ~350 kg CO2eq excluding running costs with a yearly carbon footprint of ~120 kg CO2eq. [Sidenote: This works out to a 100W computer running 8 hrs/day for a year. Yay for sanity checks!]

To match the carbon footprint of a desktop computer across its 4-year lifespan excluding power usage, you would have to go through 70 spools of filament. This is 1.5 spools a month or printing 1.6 hours every day at the feed rate you provided. Looking at your typical 3DP user, this actually sounds pretty high to me.

Now, the last thing to consider is carbon footprint to actually make a 3D printer! I don't have that kind of ambition except to say that Epson 2D printers without consumables or electricity seem to come in around 30-40 kg CO2eq. For a 3D printer, let's just say 100 kg CO2eq because they weigh about the same as a desktop but with more raw (fewer processed) materials and they're much, much simpler than a desktop. Metal rods, fasteners, the 85g RAMBo board, maybe 1kg of plastic and some steppers/hardware. For reference, raw steel is 1.8 kg CO2eq / kg.

So now we're down to just over 1 spool per month to keep things even. But this assumes you actually keep both for 4 years. Change both every two years and you could be printing 2 spools per month (>2 hours per day) just to break even. Also, ship by ground and you can print 10% more.

So I retract my "order of magnitude" comment and instead say that this whole exercise was pretty much a wash. I didn't mess with any of these numbers and it actually just worked out like this (and yes, nice numbers make me very suspicious too but I've been over this a couple times).

Meet in the middle maybe?