Steve Blake, much like Fisher, is a good defender as long as they aren't guarding someone with elite speed. The problem is that every year, more young speedy PG's seem to be entering the league.
Let me be clear. I don't hate the Blake signing at all. I actually think he's a good player and they got him at a good price. Blake will be better for them in the regular season than Fisher has been the last two years. The question is whether or not you can replace Fisher's leadership and clutchness when it comes time to win playoff games.
In the NBA you have a short window to compete for Championships. If you have a team in that window you don't try to nickel and dime key components of the formula. And Fisher has been a key component without a doubt, and an underrated component. It's a two year contract, not a 4 year contract, it's not like it kills your future flexibility. Now is not the time to screw with that team.
However, if it means that Fisher doesn't come back, the Lakers did get worse.