A lot of comments here seem to assume that the goal of PP is to provide a complete solution for the plastic waste problem. It's not and it doesn't have to be. I may be off base a bit but I think the amount of plastic that is actually being recycled is only around 10% on average (with substantial spikes in both directions if you look at it on a country basis). So there is more than enough headroom to do something useful with the stuff that would otherwise have been burned or buried.

I kind of sort of agree with your points about the manual machines but Aliexpress was not as prevalent as it it is now 10 years ago and it's not as if there were already lots of open source blueprints for injection molders, extrusion machines and the like (unlike for example in the 3D printing space), so the knowledge built up and shared by PP is still useful.

I guess it's also a pretty human thing to want to tinker with making your own machines and share them (if not, there also would not be a bajillion static site generators, notes apps and whatnot in the software world).

I don't really agree about your points about the industrial injection molding machines, though. Sure, if you want to produce many of the same parts, that's ultimately the way to go. However, if you want to do small-series stuff, experiment with different ways of making molds, do educational hands-on stuff and so on - which is what PP is much more about than maximizing output - a manual machine is much more appropriate.

Anecdote: I regularly had people from (university) departments who have all sorts of professional injection moulding equipment use our - in comparison ridiculously primitive - setups. Because it was much easier, much faster and the "vibe" in a Precious Plastics lab is usually also very different than in a research lab (and that's also not to be underestimated if it's more about development/experiments/learning than about maximizing output).

Also: Fucking around with old hydraulics can be a rather dangerous activity and requires some safety considerations not relevant in manual machines.