The plastic has already been produced, so it's already an environmental problem, this repurposes it, one way or another it's going to break down and end up somewhere.

Plastic in a properly designed landfill doesn’t end up in people’s bloodstreams.

The company operates in Africa, where all of the plastic is just lying around.

Properly designed landfills don't exist in most of the world (Africa, Asia, most of South America).

To me this sounds like people in most of the world are getting needlessly poisoned with microplastics when a technological solution exists. Why do I doubt there is going to be a successful non-profit helping build good landfills?

Simple solutions aren't sexy enough. Heck, proper refuse collection service (garbage collection in the US) would be a start.

See this report: https://wedocs.unep.org/bitstream/handle/20.500.11822/21134/...

Rapid urbanisation and comparatively high poverty (lack of access to clean drinking water, so plastic bottles are used) lead to more and more waste without proper waste management catching up.

Almost all plastic in the ocean comes from 10 rivers - 2 in Africa (Nile and Niger) the rest in Asia [0]

[0] https://www.dw.com/en/almost-all-plastic-in-the-ocean-comes-...

It will break down much more slowly as a solid object than as separate pieces of plastic. It will likely be painted or covered with a facing anyway.

Quality weatherproof paint is expensive and needs to be maintained. That’s going to have an impact on the cost.

Plus if you ever want to modify the building you’ve got an environmental hazard on your hands.

Almost certainly.

Yes, which is why this is a bad idea.