In the simplest terms. I trust IBM to maximize overall return-on-investment (e.g., profit), and a "free" CentOS that truly competes with licensed-for-fee products does not fit that for-profit model.
Whilst I don't disagree that one should be cautious, it seems to me to be strongly in Red Hat's interest (and thus IBM's interest now) to maintain a free-to-access distribution that whets people's appetites for the paid version. This could change but, for the
time being, it does seem to me that the profit motive works in users' favour with this particular open source operating system and its ecosystem.
Also, while Red Hat remains open source (which looks very unlikely to change) then there is nothing to stop another group replicating what CentOS, SL, Oracle and others have done. I admit that doing this from scratch today would probably be much harder than
in the past but it's still technically feasible. Of course, to make it realistically sustainable might still require a profit motive and business plan of some sort.
Having said all that, I must admit that I can't see why Red Hat (and now IBM) really needs CentOS. If they are happy to make CentOS available for free why not just make Red Hat available for free but without the support?