TPM Reader BM makes the same point and another that hadn't even occurred to me.
Reading articles like this, what no one will acknowledge is that if anyone did try to come in at the last minute, Romney's money will make sure they are damaged goods in some form or another. If he has shown anything, it is that Romney is willing to spend alot of money to rough up his (financially weaker) opponents. Late entrants to the race would be weak financially. Romney would tar them in the media. The result is that they would lose their chance to become President.
No, the smart potential candidates are going to sit out 2012 and wait for the current field to clear out and go against a brand new Dem in 2016. It makes the most sense for them.
Tough beans for the GOP, though.
The last point is unquestionably true but I confess it hadn't occurred to me. If you're really a gold standard candidate, you're going to step in to steer the half-capsized ship coming out of the epic crap-storm of a brokered convention?
So not going to happen.