Guys… please… take the time and find out what Purism is actually about, maybe then you can understand what they do and hopefully appreciate it.
Yes, there are way too many distros and Pop!_OS is probably just a useless Mee-Too distro.
But PureOS is not about just rebranding something.
Purism was founded specificially to make it easy, even for non-technical people, to run 100% free, privacy protecting systems. This statement alone should make it clear that Debian is not the way to go. If it does not, have a look at the Purism freedom roadmap.
It might surprise you but Debian is actually not endorsed by the FSF. Here’s a list of endorsed distros, which is surprisingly short.
Purism strives for FSF endorsement for all their hardware and software, and therefore for PureOS 3.
Now, with the upcoming Librem 5, PureOS will also become a mobile OS, with built-in convergence.
Purism has an upstream-first philosophy, so all general improvements will sooner or later be part of other distros. But some modifications like a privacy-by-default browser might be to specific to be adopted by others. You cannot - and should not - impose your philosophy on others. Therefore, I think they made the right choice.
Also, that freedom Purism is striving for includes the freedom for you to replace the default operating system with whatever you like.