[turndown]

Most of the stuff I have read on this presupposes that some kind of phase transition(think of the early universe being in a 'boiling' phase and then condensating) occurred that caused the field which drove inflation(with the force carrier called the inflaton) to decay and release all the energy in the field(that is, the inflatons decayed). This decay process is what we conceive of as 'the big bang', as in the start of the energy dense Universe we see a glimpse of in the CMB.

You are right that the goalposts have been moved. When analysis of the CMB began, it was noticed that it was far too uniform in distribution and temperature for what was previously thought to be possible. It was at this point that an inflationary period was tacked on before 'the big bang' because that was the only way to get the kind of 'big bang' we seem to have had.