>People don't write their own open source. Expertise and economies of scale are a thing.

True, people don't write their open source, but what distinguishes open code from open blueprints is that in software the plan, ie. the source code is the product, only in binary form, where with an open blueprint or schematic the product is only defined by the plans. The point being that having a good plan cannot ensure a good end device, especially if the opposite is incentivized.

>I'm fine with buying my open source appliance from Walmart, as long as I can get parts from everywhere

This assumes that the parts from everywhere are not themselves low quality, which may be a fair assumption since they are parts not a product. But if this becomes feasible, and the manufacturers are indeed incentivized to make broken products, they will may not produce open source products at all rendering the existence of open source plans useless, although the OP assumed the existence already. So yes in the event that we did have open source appliances planned obsolescence might be solved or reduced by your method, but the fact that planned obsolescence exists in the first place is a signal that manufacturers will never produce them if this is the case. It is then in effect almost (in this regard) the same as the OP saying "I wish manufactures would not implement planned obsolescence." A potential solution for the manufacturer would be to simply make the most expensive part be the most the one that breaks, or even better to pull an apple and solder everything together such that you can't ever fix it. Even if the latest apple laptops were completely open source you wouldn't be able to upgrade many aspects of it. My point here is that if we start from the point of assuming manufactures will implement planned obsolescence then they will continue using it. Which is why the GP suggested that via 3d printing the user become the manufacturer or to do so with small scale manufacturing, which as I pointed out comes with increased cost and safety concerns.

>and download a manual that tells me how to fix everything that goes wrong with it.

Open source doesn't automatically suggest a manual and closed source doesn't automatically suggest no manual (as someone else pointed out, manuals used to be more common). That said, the chances of there being a manual for an open source appliance are probably very high if there are enough users or if the creators cared to document it. And of course if you can read the source than open source in fact does imply a manual, but at greater cost (which still is better than nothing of course). Either way I think my above point stands that if we assume manufacturers are implementing planned obsolescence (which the OP assumes) and if we assume that open source would solve planned obsolescence, then we have no choice but to conclude that no manufactures will make open source products. The exception being if its so difficult that only a small number of users will actually be able to repair the product or could only do so at a high cost. In which case there would be a small segment of people who would benefit. But that severely limits the statement of the OP, which is totally fine, just something to take note of.