Even if totally P2P, centralized exchanges do something way better than general p2p. For example, what use do I give to crypto? I get paid in crypto and sell to send money to my parents in my home country, not being from a first world country, finding people that buy bitcoin with my home countries's currency is pretty difficult, and decentralized exchanges really go after main FIATs. So in my particular case I find use in centralized exchanges, at least until a more decentralized option makes my home country's currency as one of the escrow-able options.
Weather it is a P2P exchange or a centralized one with a middle man, Either way you still turn your bitcoin into fiat. Now Localbitcoins used to be the way to get bitcoin and then exchanges started to appear and people started to lose a lot of money because of "hacks". take mt.gox, bitfinex, poloniex, quadriga for example. You give your bitcoin to them it is now theirs, in a p2p exchange it stays as your until you trade it. It is not constantly in someone else's hands. It is like giving your money/work/wealth to a bank and trusting they will hold on to it, and then stuff like this happens
As p2p exchanges start to open their doors more, It wont matter if there is someone in your country to buy it with fiat. You will be able to buy it anywhere in the world for anyone willing to sell it and obtain more fiat.
It will be like https://localbitcoins.com/ just in a wallet or client like the bitcoin core. Where trusted users and money changers will dominate the market.