I wouldn't want to argue the point about madrigals, ballads etc, but I suspect it may be a bit over-egged in what you've read. I think there were other reasons why the Beatles were as successful as they were so quickly in Britain.
First you have to look at the contrast they presented to their British predecessors. Most were the British equivalent of the Philly "teen idols" - which doesn't necessarily mean that some of them didn't have talent and some weren't producing pop records that were very good indeed, as pop records, y'understand. But there was a clear sense that those guys were there for their looks. (The same was true of the female singers - Elkie Brooks is the same age as me and, at 18, my tongue was hanging out when she was on TV, even though I knew that, by any standard, Etta James' version of "Somethin's gotta hold on me" was THE version.) There was a definite sense that the Beatles weren't there for their looks - though lots of girls liked Paul. But they were a tailor-made creation for the market - in much the same was as I think late sixties Miles Davis' image was tailored to the market. Their clothes were extremely Mod. And they were purposefully irreverent, which was another thing people in Britain were coming to then, with Private Eye magazine and "That was the week that was" - and coming from the Left; I dont think it was a coincidence that Labour won the 1964 election (not that the Beatles affected that outcome, but Britain was moving in that direction anyway and they were part of it).
Second, it was noticeable that the Beatles songs were hard to sing. This is, I thnk, how a layman like myself - and many others - would describe the exegesis you made on the chord changes earlier. You could sing along with the records. But try singing them as you walked down the road! (And that isn't true of folk songs, ballads etc - those are, and are SUPPOSED to be, easy to sing.) There was definitely some fascination with that; the Beatles were perceived to be interesting.
I think that trying to pick out some elements of what made them successful is kind of fruitless - they were a whole package, made up of bits that were natural talent, education, style, political consciousness, attitude and so on, and with no apparent contradictions between these elements. I mean that everything contributed positively to the end result.
MG