So I can see this change having a big effect on consumers. If by "how big a change" you meant "would anyone even care", I think people will care, yes. Including me.
How big a change was it to implement? I don't know.
Note in addition to not having page breaks, it appears to have several "responsive" features added too (from the OP description, I haven't played with it yet myself). Lines wrap at whatever your screen size is (including zoom level), and there is apparently some screen-size-responsiveness to at least some images too.
I couldn't say how difficult this was to implement, having no idea what the code is like, and knowing that large legacy codebases can make naive predictions of how difficult a given change might be unreliable.
But I don't understand why.
So yeah, I welcome this change big time.
Limiting my docs to a IRL format doesn't make much sense to me, page breaks make no sense, with H1/H2/H3 etc you can just navigate the doc that way, and internal links work etc. No need to say "check out the flurple widget subsection on page 92" you just slack/email them the link to the subheader or H3 or whatever and bam they're there reading what you need them to look at, similar to markdown docs on github, but with all the manual formating GUI'd away.
Seems like it's touted as an innovation, but the only thing I see is that page breaks are gone.
Which isn't bad, I mostly use Google docs for online articles and to maintain a todo list, so things are now a bit cleaner.
But it doesn't seem like a big change...