Political resistance
The TPC and its defenders point out that they have been conservative in their analysis in that they have set the bar low for the Romney plan - only testing if it is mathematically possible. Given the popularity of many of the concessions many of the mooted changes would be hard to do in practise.
This point was made in an interview with John Diamond, the author of the growth simulations relied on by the Romney campaign (see citation). Diamond argues the only way such unpopular concessions would be politically feasible would be if the candidate had a clear mandate to do them - which given the lack of detail will clearly not be the case.