In the end, I think the other comments are right that it mostly depends on who's working on something. If it's different teams, then different repo's probably make sense. But if I'm responsible for both the back-end and the front-end, they're usually not isolated at all at least in terms of project requirements, and hence keeping them together makes sense.
(But of course, even then there's nuances. I think the article is mostly arguing against monorepo's as in company-wide monorepo's. I'm willing to believe Googlers that it works well for Google, and I'm not in a position to claim what it'd be like for other companies. Team-wide monorepo's for different parts of the same project, however, make a lot of sense to me.)
It doesn't. In my entire career, that was the only environment in which some random would break us and we couldn't do anything about it other than hope for a rollback and then wait for hours for the retest queue to clear before we could deploy anything at all.
Maybe not all the time, but you need the escape hatch of pinning healthy deps, because HEAD of everything is not guaranteed to work.