"I don’t think it’s very likely nowadays that an agent would a) take on a novel that obviously needed a lot of fixing in order to be publishable, or b) keep sending it out year after year when it didn’t sell, or c) carry a client who showed no signs of ever producing another book."
This caught my attention because the context (by implication) seemed to be that agents who have done, or continue to do this are somewhat above and beyond. So, I gave it some further thought....
a) - Done it. Though that may depend on definition of a "lot of fixing." I have in the past put in a lot of editorial effort with books (including my infamous - drop the first 100pp off this book and I think you might have something here - which actually has occurred more than once). And I still do it. I (occasionally) read drafts for clients - particularly if they're attempting to tackle a new writing challenge, evolve their craft in some significant way, or change their career direction (e.g. enter a new genre). And I'm right there to offer them another viewpoint to compare with what their editor might say. Do I do this for a first novel? Very rarely, but yes (and probably not so often as I once did).
b) - Done it. The first book this happened with took me two and a half years to sell. And I swear I started to think it was under a curse. Every editor I sent it to either resigned or was "right-sized." But I knew it was a good book. I was certain of it (even though my boss was not so much). I also knew that (at the time) it was a risky book, though the subsequent development of the market seems to indicate I was at the forefront of a curve. In any case, it finally sold. And that author currently has seven books in print, and three more on the way. And -- *scans submissions list* -- still doing it for other clients. What can I say? I'm stubborn.
c) - Done it. And currently doing it. Though - admittedly - I don't always do it. Each author is an individual case. And there are times you just come to realize that your goals and the author's goals have diverged too far.
I know I'm not the only one.