It'd be easier to take you seriously if you weren't suggesting Aiyuk is a significant loss.
The Niners consistently do more with less than any team in the league. They won 12 games with a lot of their key guys hurt, and most significantly, they have, unquestionably, the best QB in the NFC.
Meanwhile, the Seahawks just had one of the luckiest seasons in NFL history, with nobody important getting seriously hurt, and winning a number of games off unforced errors by the other team (Steelers, Cards, Rams). These things have a way of regressing to the mean.