I think that an all-topics firehose version will be preserved, but it will likely be worked into the site in a way that is demoted in prominence (e.g. requiring an additional click to get to).
The preference goes both ways, some niches really don't want to be in the firehose. The default won't be able to be the firehose for that reason, preserving the option but fundamentally changing the site culture.
What I mean by the cultural shift and how the niche's preference might be to avoid the teeming masses is that the desire to see and comment on everything can — in some cases, I am not speaking of anyone in this thread in particular — end up polluting niches with irrelevant comments. As an example see most attempts to discuss programming, for example, which usually gets a response from a non-programming forum member merely saying they don't understand a thing and making a2k a place that can't currently support that niche. Philosophy threads are another more prominent example, while the ability to filter them always existed it was not prominent enough on the default to custom behavior spectrum to avoid the conflicts that followed (where each philosophy thread had to contend with a bunch of people who didn't like the threads but also wanted to see all).
There is going to be a fundamental culture shift from seeing all to seeing what you opt into. We can preserve an option to see all, but expect it not to be as prominent. This is very speculative, as I am months
away from beginning to define the UI design but what I am talking about is something along the lines of having the all-topics firehose an additional click away. The additional click is a bit of friction we are putting up to seeing all, as the desire to have less monolithic microcosms conflicts a bit with the desire for the firehose and as the site scales the firehose will simply become less and less useful, making the preference for it something akin to miswanting (i.e. wanting something that is inevitably going to have a diminishing rate of return).
As the site grows, the value of the firehose for any particular individual decreases. Don't just think of it as topic interests but sheer volume. E.g. if you had a feed of every post that was ever made on facebook or twitter it would be very interesting but it's not humanly possible to even read 5% of them due to sheer volume and the grid would move too fast to do much but a random picking of what you want to read (e.g. by the time you read the titles hundreds more would have been posted etc). So the fundamental paradigm needs to change from opt out to opt in.
This is a long way of saying that you will probably have what you want but it will probably be less easy to get to because what you want isn't great for the site's culture. I want that option too, but I think it should be relegated to a secondary way to browse the site instead of the primary one it is now.