I am going on the number of occurrences in which the circle I know have encountered steelhead on the per each basis going after whitefish and cutts. I stand by my initial assessment. We also happen to be fishing rivers like the Stave involving winter runs, high pressure, etc and not the summer runs like you do on the island.
I'll start off by saying that I gave you a winter run example. The river example I gave is on a river that I guarantee see's much higher pressure than the Stave, along with much higher bait usage. It's one of the busiest on the island. The other busiest river - the Cowichan - also sees winter runs commonly taking stone flies when guys are fishing for trout. I quite commonly fish for mainland summer runs too, as I spend most of my summer and fall working there. In fact, most of my true summer run experiences are on the mainland and not the island (not to mention on "busy/high pressure" rivers as there aren't many mainland summer flows).
The main thing to remember here is that you are targeting totally different water, in rivers that have significantly fewer numbers of steelhead than the Vedder. I wouldn't expect to catch a steelhead in most of the sloughs, ditches, lakes, etc that you, or the people you know fish. While there are chances at steelhead in the Stave, I would argue your chances of encountering steelhead in the Vedder, Chehalis, etc are much higher (and surprise - I rarely fished these for trout, or knew anyone that did!). Again, it's the water you fish that determines your steelhead by-catch.
My point is - your description of "a steelhead" (you didn't specify summer nor winter) taking 1 nymph per year is grossly exaggerated, and it's mainly due to your lack of fishing for them.
You forget, that for a good number of years, all I did was fly fish for cutties on the lower mainland. Again, probably 80% of the time I spent fishing was on waters that did not ever have steelhead (sloughs, small creeks, ditches, and lakes), so I wouldn't have expected by-catch. On times I did fish flows with steelhead, I would get them occasionally. Most cutty flies are regularly taken by steelhead. Many steelhead love eating nymphs, alevin and fry. I rarely ever used anything less than a size 10 for cutties, and therefore, that's what I've caught most of my "trout fly" steelhead on, size 10-8 streamers and nymphs. The ONLY stone flies I fish are size 10, no need to go bigger for steelhead (even when I'm targeting them, both winters and summers). If you're only fishing for whitefish, right on bottom with tiny nymphs, then of course I could see a decrease in your by-catch. The initial question was about steelhead taking trout (rainbow/cutthroat) flies, using trout tactics - not whitefish flies fished right on bottom.
Plus you're also forgetting the original question:
Will rainbow trout take steelhead flies? And vice versa?
The answer to that is yes (mainland wasn't specified, and it shouldn't need to be). Steelhead DO take rainbow flies all the time (they are just a big rainbow, that still eat and have trout preferences!), and I quite often catch rainbows on steelhead flies too (you can see the intruder tucked into its mouth there, a blue and black):