The 49ers have an expensive defense, and are starting to struggle with keeping it together under the salary cap. Their Pro Bowl safety, Dashon Goldson, was not franchised, and will very likely find greener pastures somewhere else next season. Acquiring a player like Revis would require two things, draft picks and a cap space.
San Francisco has plenty of draft choices, but not plenty of the 1st and 2nd round picks it may require to get Revis. Any time a division rival surrenders high draft choices for a high-priced veteran, it is a good thing.
The cap space Revis requires would have an impact on who else the team could add via free agency over the next few seasons, and possibly require them to shed more talent from the team. Even if it is only loss of depth, it is also a good thing.
Guaranteeing big money to Revis would marry the team to him for the foreseeable future. This is a player that missed most of last year due to injury. Revis has not been an injury-prone player, but another injury while taking up significant cap space would have a dramatic impact on the 49ers.
It is not, however, all risk.
Revis would make the 49ers already potent pass pressure more effective. It would significantly improve their ability to compete against top-end quarterbacks that have given them trouble at times (e.g., Eli Manning, Joe Flacco). Vic Fangio is one of the best coordinators in the NFL, and would have a field day designing a defense that only had to guard half the field or could ignore the opposing team’s best receiver. The debate would be how much better the defense would be with Revis, but without Goldson and Carlos Rogers.
Seattle’s offense would largely be unaffected by a shutdown corner on San Francisco. Seattle has attacked the 49ers defense on the ground. When they have passed, it has been to a variety of receivers and tight ends. There is not a clear reliance on a star receiver or a potent passing attack to win. Imagine Revis holding Sidney Rice to 1 catch for 8 yards. Is he also going to keep Marshawn Lynch from rushing for 100+ yards again? Keep Doug Baldwin from scoring another touchdown against SF? Guard Anthony McCoy and/or a third receiving tight end threat? Will Revis keep Russell Wilson from scrambling for yardage?
Revis is a true talent, and would make San Francisco more formidable in some ways. The cost of acquiring him and signing him would reduce the 49ers flexibility and make it less likely the team could address other needs. That is part of why it seems quite unlikely the 49ers will be drawn into the trade. Do not fear Darrelle Revis in a 49ers uniform, Seahawks fans. Seattle is built to beat San Francisco with, or without, Revis, and his addition could very well lead to a more trouble than it is worth for their division rival.