I will argue and agree to the fact that centralised exchange somehow aided the easy access to Bitcoin spread globally. Actually, it also aided in the quick purchase because in most locality you barely see people who will want to exchange via p2p so the only way they can do that is via centralised or decentralized exchange which is easier to access through the internet. In my own research, I found out people prefer trading and buying Bitcoin through exchange than doing p2p. When I inquired about their reasons for doing so,  they told me that they preferred it that way for security and safety reasons as to the large volume or amount of purchase, they said centralised exchange somehow is safe and secured for trading as they do conduct kyc so therefore they are ready assured of their funds and assets. There were lots of reasons mentioned by them but just these few I could list do to cut the long story short, I think OP topic is a good idea and calls for a wide range research.