Rental not Retail
Kindle unlimited is rental not retail.
You get access to any book on the "unlimited" list while you continue to pay the monthly subscription.
Stop paying you lose access.
Paying by pages read might make sense in this model.
However my personal choice is to pay for each book I choose to read. I seem to be reading a lot of the self-published books at the moment because the buy price is generally pretty low (below my why-not threshold)
On the whole I don't think I'm paying extra other than when I really get into a book and go through the 7 book series. In which case I'd rather the money went to the author.