But back on topic:
That's the so called "resin rot", that will affect 99.5% of older models. There's no way to avoid it and the only remedy is to get a new set of resin. However, I wouldn't use soap, neutral or not, very often on an old model. Get it clean once and baby it afterwards (use it sparingly). If you frequently use soap to clean it you will accelerate the natural decaying process of the PU resin - the same goes for sun or UV radiation exposure.
I thought that resin rot would always be an obvious thing. The telltale sign that resin may be suffering from it is a layer of "dust". It's actually particles of the resin breaking down. You can wipe it off and see the tissue having the color of the resin on it. The resin surface becomes shiny. Once the cracking starts, it'll just surprise you. There won't be much warning. A hairline crack will show up and then any pressure on it will cause the resin to separate.
I recently got an old DW-1100 without a band, and found my backup NOS TW-7100 resin band fit perfectly. There was some resin dust on the band, which I wiped off. But a flex test showed it to be in great shape. So I installed the band and wore the watch. Later that day, I'm sitting on my couch and while moving my arm the watch falls off my wrist. I discovered that one of the straps broke into two pieces. A clean separation, right through one of the tang holes. There was no warning, no sign of any problem. After this, I flexed all of the remaining bits--solid. No further cracking. Just this one place. A real shame. I wish it could be repaired, but I don't think there's any feasible way to fuse the resin back together.
I have to say this totally surprised me and completely changed my outlook on vintage resin. My hope is that newer resin has better formulations that will last for 50+ years. But that older stuff? It's an "any day now" proposition for failure. I would never buy a vintage DW-5500, unless at a bargain price (and then flip it for a profit). And now I'm tempted to sell off my older resin while it's still usable.