Making a coaching change doesn't mean you have to wait another 4-5 years to be good - particularly if you have talent on your roster. The 49ers changed coaches from Singletary to Harbaugh and went to the NFC Championship in the first year thereafter (and were a muffed punt away from going to the Super Bowl).
For that matter, installing a new offense doesn't have to be a 4-5 year process either. Martz came in in year one and installed an offense that only the would-be starting QB, Trent Green, had any experience in. We know what happened to Green, which means our eventual starting QB that year had to learn a new offense as well. Thus virtually every player was learning a new offense. And yet, right out of the gate they were able to run it effectively (with the word "effectively" being an understatement of epic proportions). To avoid getting this discussion sidetracked, I am not suggesting that someone can come in and turn our offense into the GSOT right off the bat. What I am saying is that someone who is a good teacher, who can relate the concepts to the players, who can implement a system that easy to learn and one that doesn't rely on excessively verbose playcalls, etc., can install a new offense that can be run with great efficiency right out of the gate.
As for the WR's, these guys were all brought in by Fisher, through the draft, with the exception of Britt who was a free agent.
The thing that makes me hesitate most about replacing Fisher is the defense. It isn't broke, and I don't want to change it. For that matter, I don't want to change most of the staff, and would love it if a prospective new coach would retain Williams as the DC.
If Fisher has to go, it's because he's gotten us to a point where he is no longer able to move the team forward. I recognize that he had a huge task in rebuilding this time when he first took over. And he has largely done a good job with that, as the roster is leaps and bounds better than it was at the end of 2011. Furthermore, I understand that he had two seasons in which his staring QB was lost with an ACL tear, which is hardly his fault. But that being said, his approach to offense is holding this team back - it is the reason we can't seem to take the next step. And given his history, both with the Rams and previously the Titans, that doesn't look like it's going to change. Thus, barring a sudden, unexpected change of approach by Fisher, it's really beginning to look like that moving on is going to be required for this team to take the next step.