I too can't understand why selling all the cars. Looks like a financial requirement doing it. Can't believe that a car nut doesn't understand that those cars always could burn. And his argument that he can't see the flames on a rear-engined car, why would you sell an AMG GT-R, then?
Regarding replacing or not. Well - there is always a way. If it would be my car and it really was burning down because it has an error in build, then hell ya - I would like to have a new car. Think of that Porsche GT3RS which been burning down some while ago. All of the cars been replaced. Those cars have been limited, too. No-one ever doubts that it would have been the right way to handle things is telling the clients "oh sorry. The car was built shitty but we can't replace it". If it is really a manufacturing problem, then replace that car. It is only a car - doesn't make a difference if there are 500 or 501 built. In reality, there will be more built anyway as McLaren needed test cars, crash cars, etc.
Sometimes those car collectors take themselves way too important. A friend of mine is an extreme collector. Has more than 500 cars. Two of each limited (as he has two sons and wants to pass it on in the heritage). So - two 918, two GT2RS, two 991R - here it comes. The moment the GT3 Touring was presented he was going nuts. How can Porsche do this to his valuable investment? He will lose money now... bla bla bla bla.... Matter in fact, the 991R never went below MSRP and in fact, the prices been stable up in the hights. But he still complaints and thinks that Porsche did something bad.
Again - if McLaren messed it up then handle it in an ethical way to the client. Give that owner a new car. Any of the other 500 owners will understand. Otherwise, you can just hope that the next one burning is at least the car of the one which was against an extra build. And if it was a mistake in build then it is just a matter of time until the next flames are going up.