Fortunately it is not a claim in the text you quoted. If a change is necessary, it has already happened everywhere. So neither t-flapping nor th-fronting are necessary, but both are more likely than [t] > [k]. There is no specific reason that one of them advanced through one country and the other spread through another, even if there is a good reason the third change hasn't happened. Or equivalently with the abandonment of “reckon” vs the substitution of “autumn” for “fall”.
Could you expand on that? Do you mean that they are predictable or possibly artificially steered?
In mainstream linguistics sound changes are assumed to be random. Some are more common than others of course, but there's no saying what sound change will happen in English next.