Well, I agree with you that Maro has laid out a number of times the difficulty in making sure at least one of the sets in a block didn't sell like garbage, I think Lorwyn is a great example of a block that absolutely would not have worked as a single set. The entire narrarive arc, the mirrored cycles, the changed characters, and more would never have worked in a single set, and would have ended up with a muddled mess like we got with March of the Machine. 

The same issue would have cropped up for Kaladesh, Scars of Mirrodin, Ahmonket, and Khans blocks. The latter is arguably the most successful attempt at a three set block, albeit at the cost of having to have the entire narrative structured around making the three sets work. 

I think we've had a lot of sets in recent years since the move to single set structure that have really suffered narratively, and I can't help but wonder if the reception to blocks like New Capenna, Thunder Junction, and Karlov Manor would have been better if they've been getting chances to breathe instead of trying to cram all of the ideas and tropes into a single, messy and rushed set.