Doing this operation require quite an effort. And you do not really know how long it will take till this root access will be fixed (you can not really trust random vendor on hackers forum, this post could be searchable through google by mysql admins, some internal audit program could detect intrusion.)

So easiest to monetize - insert malware to put trojan on visitors machines. Next - hack in to their bank accounts, or use these as part of bot net or whatever. You basically got highly visited place to put classic malware.

The point is... To do such things as replacing mysql source flawlessly is hard, do not underestimate efforts needed to do that.