Disease may be killing the fish, but it's not making them smaller. Gillnets may be making them smaller on the spawning grounds but not in the river.
Neither of these is making them younger.

marine mixed stock fisheries on non-adults-ocenan sport, ocean troll, bycatch in trawls, squid gillnets, and such is what is taking away the old fish.

The reduction in Chinook size,and the causes of same, were recognized when i was college (Noah was my fisheries prof).

The first step to recovering the Yukon Chinook size, or the Elwha Chinook size, or the Skagits, or anywhere is to quit killing them in fisheries until they are mature and stopped feeding. Then, of course, letting enogh of them spawn to support the run.

Management generally sets escapement goals as number of fish. As the females get smaller, thry bring in fewer eggs. An EG of 10,000 may have worked 30 years ago. Today, hitting 10,000 may give 60% as many eggs.