Anything biodegradable (in the composting sense) almost always does not last long absent significant chemical treatment. If you think current faux-leather technology is a step down from the real thing, then mycelium based products will be even worse. There is an inherent trade-off between biodegradable and long lasting products. What we can do is make products that are recyclable. In other words, engineer materials to only break down with a specific brand of artificially engineered enzymes that are not found in the wild. Unfortunately we are still a couple years away in terms of proteomics advancement to be able to do de novo synthesis with such precision.