Plastic that disappeared after 50 or 100 years would still be very useful for making things. Plastic for packaging only needs to last months or years.

You need to avoid certain kinds of biodegradation but you don't need "forever".

And in particular if you could make very small pieces biodegrade quickly, and only those, you'd instantly solve microplastics.

That reminds me of a fun research presentation I saw a decade ago. They engineered a bioplastic which would break down on sustained contact with water. While trying to come up with applications, they proposed reducing plastic consumption by using it for ... water bottles.

There's a bit of nuance here. A material which doesn't biodegrade but is inert, a material which biodegrades but only when exposed to light or moisture or heat, a material which biodegrades into something else which is in turn inert, etc.

Not really relevant to geocells but quite relevant for e.g. timber, which might last a hundred years in a roof but start to rot in six months of composting.

The problem with plastic is that it often doesn't biodegrade but loses capability with time. So you end up with it turning to rubble after few decades but still polluting the environment