The worst that could happen would be that Red Hat would stop shipping sources for all the BSD/MIT/etc licensed bits, and be as difficult as possible for GPL sources (written offer, valid for three years, and making CentOS pay for the media costs, and then play stupid legal games with the requirement that it be `on a medium customarily used for software interchange').
They don't strictly have to put all the sources in a conveniently compilable format on FTP servers.
Oh, there's worse things Red Hat can do to block access to GPL source. For example, technically they only have to provide source to their customers, and there's nothing in the GPL that stops them terminating the contracts of any customers who distribute that source code and closing off their access to updates.
In a way they're already doing that with kernel source; the actual broken-out patches applied to the kernel are only available to customers and they're contractually obliged not to distribute them otherwise their contract will be terminated and they'll lose access to support, software and security updates and the right to run RHEL at all.
That's a pretty novel interpretation of the GPL, and I doubt that it would hold up in court. Reading through the GPLv2 I can't see anything in there that even suggests Red Hat would be within their rights to do that.
They still provide all the sources, as well as easy means to build them ( a source rpm ). This is far more than what the GPL requires them to do. What they don't distribute is the srpm with all of the patches neatly broken out by bug number and CVE (if applicable).
It makes it harder to figure out what sources came from RH, and what are vanilla kernel sources, but there's nothing in the GPL that says you have to detail the provenance of every line of source code distributed
They don't strictly have to put all the sources in a conveniently compilable format on FTP servers.