The rigs already have a Plan B and a Plan C. The problem is that they didn't work, and nobody knows why yet.

Deepwater blowouts are rare and it's not clear how much actual real-world experience there is with the seafloor safety systems. Adding more unproven backups may actually hurt by leading to complacency: "we've now got Plans D, E, and F: what could possibly go wrong?"

At least one of the blowout cap systems on the seafloor is destructive (to the well). It might be best to focus on mechanisms that can be tested after installation and periodically during well operation. A safety system that is both destructive and lacks significant real-world usage is not a good thing to rely on.