It's not going to replace exisiting SAR, but it will augment it.
Does it actually need augmenting? This is a new(ish) toy that someone needs to invent a job for to recover R and D costs. If it can only go out on nice days it's really not much use at all.
Perhaps you should go out on a real search Nige321 before coming on here and calling us luddites.
I did the first job with the IR/TV turret on the RAF SAR Sea Kings - it was a search at night and we would not have found the guy (badly injured on a beach after a cliff fall) without the FLIR but we couldn't have saved his life by getting him to hospital quickly without the helicopter.
A drone might have found him but then what? Oh yes, call the helicopter which could have been there in the first place.
Each time you think you find something on a search, you have to investigate it closely to determine if it is who or what you are looking for - is that going to be the drone's capability? It usually requires the human eye and brain combination to work out what your sensors have seen.
Your drone is very likely to identify a number of false positives that need further investigation - then you have to drag another asset away from their search area.
Nice idea Nige but learn a bit about searching first.