That's Bitcoins problem now isnt it? Bitcoins privacy issues are it's own. Has nothing to do with transactions made on Monero.

Second, you actually can do this thru p2p txns on Bisq, or atomic swaps via UnstoppableSwap or BasicSwapDEX. Several more on the way with Haveno and Serai later this year. Better than even coinjoins as there is no way for any third party to know a swap even occurred by looking at either blockchain. It's obvious to see when Bitcoin has been coinjoined.

Also, it doesn't have to be Bitcoin. 

You can buy Monero directly with fiat p2p with several options above, or LocalMonero/AgoraDesk. There are also cheaper and more private crypto you can go thru like Litecoin and MWEB.