Would Sanderson, Abercrombie, Martin, Jordan, Rothfuss, Erikson, etc. have failed to publish their first books successfully in today's publishing world? Or would they publish with rubbish sales numbers and fail to follow through on book 2? I'm skeptical. I think the high end is probably safe from the publishing chaos. At the margins, the least confident gambles made by publishers, publisher economics are probably devastating.
Song of Ice and Fire didn't move that fast to begin with. Gardens of the Moon was picked up with the request "Write some more of the series while you're at it" - that virtually never happens these days. I think it is possible neither series would take off today. Or that they tell them that they don't need more Epic Fantasy and that they don't want to confuse the market by pushing a guy as EF when he's known for vampires.
Another is that since the 90s and early 2000s, the sheer quality of epic fantasy has improved considerably (imo). Gone are the days of easy, cheesy epic fantasy, with 100% recycled/"borrowed" tropes, homogeneous casts, romanticized medieval Europe, and pseudo-biblical Evil as a personified force. Not that weren't plenty of top-notch books, but I think there were tons more mediocre books than today, or at least they were published and marketed more successfully. Each new innovative book carves away at the limited pool of ideas that readers might be interested in exploring, making it harder and harder to find something unique and interesting to write about. Many of the low-hanging fruit of good ideas wrt world building, characterization, and magic systems are already picked. I think these trends are probably more impactful at the high end than the economic struggles of publishing companies. We'll still see works of 10+ years of part-time dedication published to great acclaim; authors who did most of the work on their own then got snatched up by publishers who immediately recognize the brilliance of the submitted work.
Aaand... yeah, we'll still see great first books from the publishers that get immediate acclaim. Because they have the biggest marketing machines. But is it something to gamble on as an author? Maybe, but its certainly not the only game in town and the big rewards are still available from other career paths. And is the current situation favourable to new authors? Not as much as it was, no.