Channels can be thought of as one of either linerar or compounding (or loops). Essentially, there are certain channels that create self reinforcing acquisition behavior, and those which do not. The distinction is based on whether or not the last step of the funnel leads to more of the first step of the funnel. ASO, brand awareness campaigns, and PR campaings are linear channels. They provide a fixed amount of growth to your business and then the output (users) does not compound. So getting a PR bump that brings in 500 users, those 500 users do not really create another PR bump.
However, paid marketing, certain specific types of SEO (like Yelp, LinkedIn), virality (successful referral programs) cause your user growth to spin like a flywheel. Every breakaway success product has at least one of these loops, and the best companies have multiple.
Another way to think about this is using a financial metaphor. PR, brand awareness, etc. are principle. Paid marketing and other growth loops are interest.
So to bring it back to your question. I think there's not a specific time where working on linear channels makes sense or doesn't make sense. You probably have to start with a linear channel at the very beginning, because even if you have a wildly successful growth loop, it still needs that initial principle to operate off of.
Where companies get themselves into trouble is thinking that they can stack linear growth channel and create a billion dollar business. That's either not going to happen or is going to happen entirely too slowly for as VC-backed growth startup. And large company projects like mobile games at large studios will also get defunded without rapid growth so it's critical there too. So I think the part about not replacing is just a reminder of this fact.