When you say the staple "failed" what exactly do you mean? Not clear to me.
Photos?
A mechanical or materials failure?
If the metal failed, imho...you're going to need to replace all similarly critical shackles and or staples with high quality parts. Buy only Wichard, Harken, Selden or similar branded items. From a reputable supplier, not ebay. The Wichard ones are usually stamped.
Imho, the stem winching eye is best NOT used for primary mooring. I think that it's intended use is winching, and even then should be at least slacked off when trailing, fluctuating loads being taken by lashings on to the boat hull.
When moored, the fluctuating loads, including snatch loads if the mooring comes up tight, may not even be in correct alignment. This must really load the eye differently than intended?
If the stem eye is loaded like this, it will not be an easy repair when it fails. Likely to damage the hull, with attendant risk of flooding?
If you search, I made a long post about mooring to a buoy, with lots of photos.
Personally, I only used the stem eye for my "storm chain", a last resort attachment. Routine mooring loads being taken by the deck side cleats via a two leg strechy rope bridle, so as to split and dampen that load further.
