I'm afraid that's a common myth. For starters, there isn't really an objective measure of how conservative or old-fashioned a language is. But even subjectively, there's not really a reason to perceive American English as more old-fashioned.
All dialects randomly conserve and innovate on features. And when a dialect is split off by, in those case, geography, they will start to conserve and innovate different features. And it's true that American English conserved rhoticity which was lost in most of England, but it did innovate e.g. /æ/-raising where British English conserved the original pronunciation. But that's just two of many more features, and neither geographical region clearly conserved more than the other.
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.
The difficulty with phonetic spelling reforms is that you quickly run into the problem of words being pronounced markedly different in different dialects in different regions, and you inevitably end up trying to declare on dialect standard and the rest wrong. Granted, the sort of people into this reform are usually happy to declare their dialect obviously correct and demand everyone else get in line.
The last time this came up on HN, the comment thread already disagreed on whether "mention" should be spelled "menchin", "menshin", "menchun", or "menshun".
A spelling reform could keep “mention” and still significantly improve the orthography were it to simply apply the rules more consistently. And when words are genuinely different, like “ask” or “of” it's not going to hurt to spell them differently.
a spelling reform cud[] keep “mention” and still significantly improov the orthography wer it to simply aply the rules more consistently. And when wurds ar genuinely different, like “ask/aask” or “uv/ov” it's not going to hurt to spell them differently.
[] although how to spell put/putt is an open question. here I have sided with northerners and decided to spell them the same.
... are you suggesting that every English dialect spell its words differently, so that we no longer have a written common language? That Americans spell "were" "wur" and Brits spell it "wuh", so that when I read a sentence written in a book I have to know or puzzle out the accent of the person who wrote it?
[As an aside, why did you drop the doubled "p" from "apply" but not the doubled "f" from "different" or the doubled "l" from "spell"?]
All dialects randomly conserve and innovate on features. And when a dialect is split off by, in those case, geography, they will start to conserve and innovate different features. And it's true that American English conserved rhoticity which was lost in most of England, but it did innovate e.g. /æ/-raising where British English conserved the original pronunciation. But that's just two of many more features, and neither geographical region clearly conserved more than the other.