The thing is that VAR wasn't culpable for Maguire's header. The referee gave the foul in real-time, I think, and once he had done so, I couldn't see how he would overrule himself with the help of any video evidence. He saw everything clearly, and was probably one of the very few people who thought a foul was committed.
Last night was a great example of the fact that VAR can help you reach the correct decision a greater proportion of the time, but ultimately human error, or a minority opinion/interpretation, and not the use of technology, is still a bigger barrier to getting the consensus right decision every time.