RVR replacements by pilots
Not an expert but think it depends on the authority issuing the ops spec for the airline. I vaguely remember that pilot assessment could be used to replace the TD zone RVR for take off but only down to around 150m ( that was EK ). But in CSA now, the ops spec for low vis takeoffs is a minimum RVR of 175m, with no mention of pilot assessment.
Other factors might be that on say, a stop limited runway where maybe you need a Stop end RVR, how would you replace that with a pilots observation (without back tracking the runway for example).
As I said , not an expert but it does seem to vary around the place.