Most plastics will get brittle over long time, at least the older production ones (modern plastics have better plasticisers) even when kept in good conditions.

Most plastics will get brittle over long time, at least the older production ones (modern plastics have better plasticisers) even when kept in good conditions. Exposing it to UV light will make the process faster. But even O2 exposure will oxidise it over time. Rubber will also dissipate over time and get brittle and crumble. All of this is usually fine from a perspective of a humans average lifespan, but a lot of artefacts from this age will not be preserved.