People will step on the path when they are walking on it. Rubber wears down paths, as does any sort of abrasive sand or rocks between the shoe and the road. There is a significant amount of wear even on stone paths from foot traffic. There are some plastics I would be willing to try, like UHMW, but wear resistant plastics like that are also very slippery and not cheap even in recycled form.

She is essentially making composite decking but in brick form. The sand filler will probably take a lot of the wear. Solid blocks of plastic last a long time so the degradation will be minimal compared to the waste plastic it is being made from.