The direct regulation for the defi platforms can be created too and it's easy for the regulators to create it. You must know that the regulation was coming from the government to give restriction for its civilians.
The tax can be charged to the defi platforms.
"De-Fi" platforms can't be directly regulated because of their distributed nature. But governments can easily hold accountable developers for their actions. That is of course, if developers disclose their identities on the public Internet. But what if developers are anonymous? Then regulation becomes widely impossible. What governments can do is try to regulate centralized exchanges, as they serve as the Fiat on/off ramps for the crypto/Blockchain world. They can prevent certain tokens from being listed or even traded on centralized exchanges. But they won't be able to stop decentralized exchanges like UniSwap or JustSwap.
Nonetheless, no one can predict what will happen in the future. If governments find a way to properly regulate "De-Fi" platforms, then it'll be an end of an era for decentralized banking. Consider what happened to the ICO trend back in 2017, after heavy-handed regulations by mainstream governments. This caused the creation of ICO alternatives which are regulatory-compliant (like STOs, and IEOs). Something similar might happen with "De-Fi" platforms if governments successfully regulate them. At least, crypto/Blockchain tech will carry on as usual because of its decentralized and open source nature. Just my opinion