The replacement is easy: Williams. The decision to do so: not so easy. It could make the team worse if Milano gets back to warrior All Pro status. It could also have cap implications that limit the ability to get other FAs. OTOH if you included him in a trade for someone like Garrett (let’s say Milano and Miller and a high round pick), it could balance out salaries, although I will defer to those much more expert that I on cap/salary stuff.
When it comes to veterans and when to pull the trigger, I always remember what Bill Walsh said: you’re better off getting rid of a guy a year too early than a year too late. Given Milano’s age and injury history he might fit that. Love the guy, a great Bill, but the NFL is a harsh business sometimes.