A viable solution to big CAs is to have all sites have their certs cross-signed by at least two different CAs. Then any single CA can be revoked without affecting any sites at all.

Note that this is also how a new CA is bootstrapped: initially the certs it issues are cross-signed by some existing CA so they work even in UAs that don't have the new CA in their trust root.

The obvious drawback is that now sites need certs signed by two CAs, and getting them to use even one CA is hard enough...