I take it you went to the Sumallo Grove access? I have never heard of the Skagit described as a "trophy fishery". For the most part in my experience, it's more a location for fly fishing not hardware though the bulltrout will take spoons etc. The trout also spend most of their life cycle in Ross Lake coming into the river in late spring then moving back down into the lake by lake fall. The fish start leaving the upper river in late summer - it varies from year to year.
Over the years the fishery has changed and many people have talked about the decline of the trout population and an increase in the bulltrout. Invasive fish have made their way into Ross Lake and seem to have upset the balance that was established after the dam was competed - Ross lake is a hydro impoundment. I fished the Sumallo Grove access a lot many years back. In my experience one has to walk down the trail quite a distance to reach the better fishing.
Have a look at this map:
https://www.alltrails.com/explore/recording/afternoon-hike-7608de7You can see the trail bends away from the river just downstream of the confluence of the Sumallo and Skagit. In my experience the fishing wasn't particularly good from mid-summer on unless one walked an hour or so down the trail to where the trail got close to the river again. From there one can fish to and down past Delacy camp. I think it is a dozen km and a days walk to 26 mile bridge which is the first access from the Silver Skagit road access. The fish are usually out of the Sumallo very soon after the river opens July 1st. In the past, once the water gets low enough it was possible to wade the river all the way from the confluence to Delacy camp.
With the river currently inaccessible from the silver skagit road I hear there is a lot more pressure on the water below Sumallo grove ...that may impact the fishing.
Hope this helps