"The fact that so many current sci-fi writers have paid homage to The Day of the Triffids and John Wyndham suggests to me that he shouldn't be dismissed just because the book seems implausible to you."
I don't know who those 'current sci-fi writers' would be, and I had never heard of John Wyndham before MN despite reading this genre voraciously for several decades. He might be a UK favourite but it would be a stretch of the imagination to assume that he is one of the absolute greats of sci-fi.
"Most very clever sci-fi sounds implausible."
Absolutely not. Very clever sci-fi is by definition logical, technologically and scientifically sound, internally consistent, and very plausible.
"Asimov, Philip K Dick & Arthur C Clarke are also post-war sci-fi writers, and they may not be the epitome of sci-fi writing but they are certainly well-respected & loved."
Having read a lot of Asimov's stuff (not only his sci-fi books but also his writings on science and mathematics), as well as Philip K Dick (almost all his books and short stories) and Arthur C Clarke (all his books, I think), please trust me when I say that they are at the very top of their league ('classic sci-fi').
Philip K Dick wrote under the influence of some very interesting hard drugs
so his books/stories are not terribly realistic at times, but they are incredibly imaginative and mind-bending. They have pretty much never been equalled.
Asimov & Arthur C Clarke were two of 'The Big Three' sci-fi writers of their time - engineers/scientists, with in-depth knowledge of the subjects they were writing about, and their stories are plausible, internally consistent, and surprisingly correct in detail. For example, all approach vectors, alignments of Jupiter's moons etc in 2001: A Space Odyssey and its sequel 2010 are correct and valid. He has actually gone to the trouble of calculating them 
Neither of them would be caught dead writing about aggressive plants taking over the world, plotting war against people without brains or muscles 
"As my DH tells me about rock guitarists - the early ones created the language the rest of us speak."
I'm a fan of Deep Purple and Led Zeppelin and would like to think I know a bit about rock guitarists of the time
Imho they were much better than the current bunch re technique & talent. Still, there were many guitarists who were not geniuses in those days, too 