We have a decent stock of owned systems and manage these via a centralised library system.
No specific issues with longevity of the nattress covers, however you do need to clean them correctly (use of compatible cleaning fluids/techniques) and also launder them correctly (correct temperature for wash and drying cycles and methods).
Not sure if you are cleaning the top covers yourself? but if you do, make sure the processes are correct for the particular covers in question.
Incidentally, damage due to 'miss-use' at the ward level is generally an issue for the ward staff to resolve and not the equipment manufacturer. We have experienced a number of such issues such as sharps damage or severe staining of one sort or another. This can render the protective none permeable layer to the cover useless which could lead to what you are describing