Saw this piece of well-worn Seiko Mapmeter SKZ227 at the watchmaker’s shop recently. The black plastic parts on both sides of the watch are not ageing well. They are broken and some parts had fallen off the watch.
If I remember correctly, this model was released in 2007 so I figure theses pictures are probably good indication of what the plastic parts would look like when worn regularly for 5 to 6 years (2007 to 2013). The first crack on the plastic probably appeared much earlier though.
From the pictures above, you can see that some parts of the plastic had fallen off and the parts that are still attached to the watch have attracted a bit of dirt.
Besides the Mapmeter, watches like the Franken Monster, Baby Tuna, etc come with similar plastic parts and collectors will face the same problem if they plan on holding on the these watches. There is no guarantee that parts for these entry level watches will be available several years down the road. One possible solution would be to buy spare plastic parts and store them in a cool dry place.
Here are some photos of a SKZ227 in happier times.