Thanks to all for the various input and suggestions, it's been interesting to consider. A few specific responses and some general notes:
Dave, I appreciate the comparison on the Ozo vs Alpha SL Pullover weight. After revisiting these two, I began to suspect a large part of the difference is zippers. Your Ozo durability use case is also really good info! I'm also looking forward to the rest of the installments in the SOTMR (esp. <8oz shells) soon. If there's durability beta on those shells that will not appear in the articles, I'd also love to hear that.
Brian, this is great durability beta on the OR Helium! And it's still waterproof after all that pack wear? Awesome.
I would love to hear more durability anecdotes like Dave's, Brian's, or (from farther back in the thread) Ryan's, about any shell, covering intensive use over short intervals or normal wear over moderate or longer intervals. (It feels weird to call a CDT thruhike a "moderate" interval, but I am hoping for a piece to last me years.) Anecdotes of good and bad durability are equally useful! I should point out that in addition to jackets, I'm interested in rain shell pants, which likely will take more intensive abuse [glissading, wading through brush] -- but perhaps less intense normal wear [no pack strap friction] -- than jackets. I do realize that they are probably less commonly used here than the shell tops.
I've somewhat overstated the abuse I expect my shell (top) to see, mainly because I've found my previous shells' durability so pathetic just under normal wear conditions. I would define "normal wear" as abrasion and stress due to packing and storage, contact with me, my clothing, and my worn gear (backpack, harness, helmet, etc.), but not pokes from sticks and brush, crampon bites, rock scrapes, etc. In fact, the large majority of the latter that I've experienced have not occurred while wearing a rain shell. My Essence only brushed through some mild branches a few limited times that I can remember. It has seen use in rain on trips on trail (or in alpine conditions with no scrapage) as well as covering me on my daily commutes by foot or bike. My Houdini, on the other hand, has withstood slide alder, slips of multiple yards down rough slopes, prickers, rough spruce boughs, etc. surprisingly (even shockingly) well. I do not even see any scrapes on the fabric.
I've been shopping under the pessimistic hypothesis that no ultralight shell is going to last years under normal wear. (My experience does not contradict this hypothesis.) However, I really hope it's false, hence my search here for counterexamples!
An ultralight shell that could withstand normal wear for multiple years would get me really excited.
A non-ultralight, but still reasonably light shell that could withstand normal wear for multiple years would still make me happy.
An ultralight shell that could withstand bushwhacking and occasional alpine stress for years would have me beside myself with delight.
A non-ultralight, but still reasonably light shell that could withstand normal wear for multiple years would make me very happy.
(Largely re: no pit zips on Helium, Ozo, etc.) As long as there's at least moderate breathable/ventable and it's actually keeping water out, I will be fine, as I have managed OK with the breathability of the pit zip-less Essence. (From Will Rietveld's Field Testing report, it would appear most options are likely better than the Essence, though the membrane has changed since my version.) I have on occasion really missed pit zips from my older shell, but mostly been OK. As long as my shell is actually keeping water out, I can manage the condensation with other methods.
The light and well-warrantied options are attractive, as Eric and Mike have pointed out, but without reasonable evidence that it's going to last, I still get skittish. From a token environmental standpoint, one of the reasons I'm looking for something to last years is to minimize my impact. I would rather buy a piece that's a little heavier, more expensive, or more intensive to manufacture and have it last me five years than buy one that's less expensive and lighter, but have to replace it (even if for free) annually. Brian's beta about his level of use of the Helium on the CDT gives me a glimmer of hope that maybe even some light options would last the distance.