Sockeye salmon do not spawn in lakes, they spawn in streams that are connected to lakes. Some populations migrate into the ocean immediately once hatched, but most populations retreat into lakes where they would spend one or two years to feed and rear.
The problem with overspawning isn't really the fear of late arrivals damaging early arrivals' redds as interest groups are lobbying (btw, why are users rather than managers and biologists discussing this in the media?). Redd damaging is what you would expect to see when everyone mate at the same time.
Whether the system is full or over in capacity, the amount of hatched fry would not differ that much.
The problem is the amount of food available for these fry. If productivity does not match demand, then surviving fry will not be as big, therefore mortality may increase when outmigration begins. That being said, should one take the excessive amount of carcasses into account when determining productivity? Wouldn't productivity be correlated to the number of returning spawners therefore food for fry should be extremely abundant when the system receives surplus broods?
This is just one of many factors that determine success in spawners, eggs, fry and smolts. The fish-hungry public may be easily convinced by one simple statement "want more fish, catch more fish", but ultimately trained individuals who study the species closely without any financial interests are the only ones who should be determining what is best for our fish.