I hope they stick around, I love the idea and right now I love Scribd. I had KU also, but cancelled that for lack of content. Scribd has many of the publishers and authors I like to read. Not brand new releases, but I see stuff a couple of years back. Considering that everything is in some sort of series, I usually have to read the back list first with most new releases anyway. I am OCD with reading in order, even if its a very faint connection. I still have to read book 1 first.
I read a lot, I use a subscription, I use the library (ebooks) and I still buy books and new releases. I break even pretty much and then some if I read 4 books a month with Scribd. Then I have my already owned books, library stuff, stuff that just comes up because of reasons, etc.
I think voracious type readers have always had many ways to get their book. I don't think its ever just one or the other.
I wish I could read Scribd on e-ink, but my Fire 6 an Nexus do pretty well with that. I would have loved KU if it had all the publishers Scribd managed to get. But oh well. I go where the content is. I want to read what I want to read, not read stuff I wouldn't normally read and just do now to break even on the monthly fee.
I know Scribd just signed a large publisher for a year contract, so I assume they are good for at least another year.