You buy Bitcoin with fiat to enter the crypto sphere and fiat is centralized and traceable. Then you put it on a centralized exchange where usually you have to get approved and share all your private data. Then you still cannot spend Bitcoin without a payment processor or an exchange unless you agree with someone that they will take Bitcoins directly from you.
That's not true, exchanges and payment processors aren't part of Bitcoin's protocol, and it's possible to use Bitcoin without them. You can do direct p2p trades to trade fiat and you can look for places that allow you to spend Bitcoin directly. I get what you are saying, but it's ambiguous to say that Bitcoin is semi-decentralized, and less knowledgeable people can see a wrong picture from it. Fundamentally Bitcoin is as decentralized as it has always been.
Come on, P2P? Have you seen the volume on those sites? Probably not even 1% of crypto users go to these sites
No one knows that volume
This is the whole thing with the P2P markets. We don't and most importantly can't know how much Bitcoin is being bought and sold there. Apart from this, even if only 1% of all crypto users are trading via P2P marketplaces, that doesn't in the least mean it is only 1% of all bitcoins being traded. The entire point of using P2P is to sell huge amounts of coins in a safe way but without drawing too much attention to your operations as it inevitably and inexorably happens when you sell or buy using regular exchanges