Merge mining already exists. It was invented by Satoshi with Namecoin. The federated Rootstock sidechain already merge mines with Bitcoin. BIP301 is just optimised standardised version of this, for multiple Drivechain sidechains. Sidechain transaction fees go to miners like mainchain fees go to miners. 

So how does drivechain break incentives?

Some users may choose to trust custdodians. Look at Lightning 90% of LN users are choosing to trust Walle of satoshi. 

Some users may choose to trust miners to follow out game theory of PoW like bitcoiners already trust miners for mainchain.