DeFi will increasingly play a role in the global monetary system, and Bitcoin will inevitably be used within it. One of the drawbacks for many holders of Bitcoin is the lack of a yield, as with many proof of stake coins through staking. I believe many Bitcoin users will gladly use a protocol that safely and securely offers them more Bitcoin. Numerous projects are working on this.
Another argument - not your keys, not your coins.
Not many out there would give their BTCs to such DEFi initiatives..
With DeFi, you would have your keys, though the BTC would be locked in a smart contract. This is not much different from having U.S. dollars in a contract or escrow. Besides, in the case of ETFs, where users actually give custody of their BTC to third parties, their usage has grown exponentially.