My 1983 side stays failed (twice) only after I abused them by leaving my Hobie 16 connected to a buoy for too many hours. The first time I attributed it to their age. The second time I called the dealer who sold me the replacement set. He asked if I ever left my hobie bobbing in the water, which I admitted to doing. He explained that they will sometimes break when they are accidentally kinked or experience some abrasion, but will almost always break after hours and hours of getting jerked every few seconds from the motion of waves. He then sent me a replacement side stay free of charge. (Saltydogmarine) Nice guy. That was six years ago. Never had another break.
I'm not going to tell you that metal fatigue is not possible. It's just not a high probability for what we are doing. What I can say is that on each occasion I lost a stay, having my mast suddenly come lose was not a violent event. As it fell, it caught air and slowly fell away from me and my passenge(s). I would actually describe it as "fluttering down" rather than anything else.
So go sail and run it hard. If it does happen, make sure you have a paddle twined in your lacing.