No one would pay to have a 3D printer fixed. Or at least not pay enough for it to be a viable business. A brand new printer can be purchased for the equivalent of a couple hours labor, and that’s before replacement parts.
This is the problem (or arguably success) of modern appliances in general. They just aren’t generally worth repairing.
People grumble about planned obsolescence but the reality is that there are people who will repair them, if you are willing to pay. But when repairs cost a significant chunk of the price of a new appliance, most people opt to replace.
Milling a paperclip is absurd and does nothing for the argument as it's just not sane.
There are many people that can afford a <$400 printer, yet at that price range it might be totally inadequate for what is actually needed. Some people are willing to not spend that money for a one/two off project in that manner, but would rather spend that money with someone that already has the right gear and experience using that gear to just produce the thing needed.
yeah - that's like PCBWay, et al. You can make boards yourself (even etch your own if you want to). But a lot of people opt. for the convenience of out-sourcing it.
And it's not even a tax write-off until you have enough income to offset expenses. (Which is nice as far as it goes but is small scale where you can write-off a few $K in expenses against a few $K in income.)
If there's already income paying the pesky mortgage, you start up an official business as a side hustle. As long as you are showing income even if at a loss, you then get to use that loss as a deduction. If it never pans out to be profitable to the point the tax man strongly suggests the business should close, you close it. In the mean time, you've followed a passion, that even as a loss, still gives financial benefit helping with the pesky mortgage.
There's doubtless some wiggle room but it's like home office deductions. Triggering an audit by the "tax man" is rarely a good idea for most people and may well cause accounting bills that exceed any marginal gains.