So I hope the replication is now a bit clearer. Sorry it got so long, I just kept going there. But I hope we're now able to easily answer your questions:

What happens if I stop following all the pubs that I am following?

The short answer: probably not much. This is because the crucial bit is that they still follow you. Your followers will still be able to get updates from these pubs, as long as these pubs somehow get the updates. But assuming that you follow at least one feed who is replicated by one pub, the odds of that are pretty darn good. In fact, if your unfollow all pubs, you will now only replicate the feeds you follow and their friends. "Their friends" (the second hop) may include some pubs, and so you would still replicate those pubs' messages. But those messages are nearly all "I now follow @X" type messages, don't take up much space, and are actually quite useful to figure out where to get updates. But after unfollowing all pubs you will no longer replicate the feeds they follow, except of course if those feeds are in your two-hop distance anyhow, because either you or one of your friends follows them. This will easy the computational & storage burden on your machine and is one of the reasons for "keeping the social graph clean" as people call it. Meaning: don't create artificial follow messages (from/to pubs) that cause unneeded/unwanted replication.

The second question you had was roughly this:

Is SSB really decentralized if we rely on pubs so much?

At the base, the answer to this has always been: yes. Simply because it would work on wifi, so no pubs are strictly needed.
However, with a global ssb community that doesn't hang out in cafés all the time (especially during a pandemic) pubs were practically necessary before we had rooms. But even then, a single pub operator could not decide to block your feed anywhere else but on their own pub. If you had 10 pubs replicating you, all ten would have to be taken down to "silence" you, and even then wifi transmission would still work and there would be no reason you couldn't use any other pub.

With rooms, this has become even easier to defend, since the rooms don't even store & forward data. They're essentially just glorified NAT traversals.

And this is not even getting into the possibilities of exposing your local client via #tor or #yggdrasil, making it reachable by anyone from anywhere.

So yes, SSB is decentralized.

Yes, that's all pretty correct. Of course there's a lot of detail that's glossed over here. For example, how many "hops" away I want to replicate is a parameter. My client can request any feed from any peer that it connects to and hope they have it. So nothing is stopping me from putting "hops": 7 into my ~/.ssb/config and start replicating effectively the entirety of what my client can reach.

how recent?

That really depends on how fast people move & mix. So these days... faster in NZ, slower in the US & Europe. 😐
I'm sure there's interesting networks research around how fast information will spread, and it is probably actually relevant to the epidemiology experiment we're all living through right now. But I'm not aware of it. I just assume that messages spread "slowly" when there's no pubs.

@Giulio Prisco I think for a room this seems fine, but I don't run a room myself. Maybe @Hendrik Peter can share some stats for his #picoroom?

@Tio I actually spoke about something like that in the parent thread of this one: %8CjpPAN...