I suppose as you have to be a Prime member to get it and - if you are - you now have Prime Reading anyway which allows you to borrow all the same books without restriction, then it's pretty pointless keeping it going.
Except it's not the same selection of books. With KOLL, it's been my experience that you pretty much have access to the entire KU library, if only one a month. Prime Reading, though you can borrow up to 10 at a time, is a much smaller set that certainly intersects the KU set, but includes other books NOT in KU as well, some fairly big names. Like the Harry Potter books seem to rotate in and out of it. Of the KU books that are in Prime, presumably they're somewhat curated.
That said, I have a whole wish list of books that are in KU but most are NOT in Prime. They're mostly relatively inexpensive to buy, of course, but I prefer to start by borrowing if I can.

I had KU for a year and found that about half of what I borrowed was mediocre at best, so I'm not keen to plunk down cash on complete unknowns. But also not keen to pay for KU when there's so much I want to read that is NOT in the program for one reason or another.
I didn't recall that you have to be a Prime Member to use KOLL; thought it was just that you had to borrow using a kindle device vs using the kindle app. Not sure about that, though, as I am both, of course; I could be completely misremembering.

* (And there's no info on Amazon about it that I can find to confirm one way or the other -- which speaks to the article linked being accurate. OTOH, I don't think there has been for a while -- it's not been touted as a kindle device benefit for years.)
I suspect that there's a large overlap between the set of kindle owners and the set of Prime subscribers. I also suspect that there is a larger percentage of kindle readers who use the app. I prefer a dedicated eInk reading device, but I increasingly here from people that they're happy using an app on their tablet so don't see the point of buying a kindle device. And I wonder how many kindle owners have been regularly using KOLL? I bet it's a really small percentage, but also probably some of the longest tenured kindle customers.
FWIW, I was just able to borrow a KOLL book for November and it said I can borrow again in December. I guess we'll see what happens next month.
*edited to correct myself: did some research and KOLL did require both that you own an actual kindle device and be a Prime member.