My boldest claim is that Flatpak is going to kill off the necessity for RPMs, Debs, APKs, etc. for most end-users.
No it isn’t, until you can build a Linux system on top of only Flatpak. And guess what you have then? Yet another distro using a different packaging system with its own opinions, just like the rest of them. And there will still be other packaging systems because not everyone will agree with how it does things. Especially once developers start including questionable code in their Flatpak packages, because nobody is there to stop them, which distro maintainers are going to strip out in distro packages because it’s harmful to users.