Just a few thoughts for you to consider regarding Forespar's Marelon seacocks.
First, Forespar calls Marelon a “polymer composite” and a 21st century material. The fact is Marelon is simply a Forespar trade name for Dupont Zytel 70G13L – BK 13% glass-filled carbon blacked 6/6 nylon (that’s not carbon fiber). The nylon 6/6 resin used in Marelon has been around for over 70 years. And the 70G13L – BK can be purchased by anyone. It’s an off the shelf material, nothing exotic.
It has a tensile strength (TS) of 17,000psi and a flexural modulus (FM) of 750,000psi. Both these values are measured “Dry as Molded” (DAM). The DAM measurement is important to remember, especially with nylon. Nylon is great in rode because in water it looses strength and elongates.
(Blah, blah, blah. Snipped for brevity)
There’s a lot more to be said. But I’ll stop here for now
This post is full of impressive factoids and numbers, but I feel that it's alarmism at it's finest.
1st- Marelon through-hulls and seacocks meet ABYC standards. ABYC is a widely accepted, reasonably safe standard to adhere to.
2nd- If you stow a 15lb. alternator, or a heavy toolbox next to a seacock, then you get what you deserve. "Stow for sea" is simply good seamanship. I never stow anything heavy or sharp near any of my through-hulls.
Everything "degrades". It's called entropy. Marelon is not perfect, nor entirely maintenance-free. It merely offers an alternative to metal hull penetrations, while presenting its own set of issues. An owner may prefer "plastic" maintenance issues vs. "metal" maintenance issues. It is however, perfectly safe and acceptable for most recreational sailing that people do.
Watercolor, you're obviously some kind of engineer. You carry what I call "the burden of knowledge". All of the facts you presented are perfectly true...and mostly unnecessary. Your burden of knowledge forces you to take the extreme, mil-spec/aerospace-industry view of everything.
There's nothing wrong with that per se, but it's expensive and often unnecessary, and simply frightens less knowledgeable people for no good reason.
Don't let "perfect" be the enemy of "good".