The key is in the name. Whoever distributes the software to you determines whether it's commercial or community. Where they get it from is irrelevant because they're the ones distributing it to you.

Ubuntu can't be made closed-source because of the licensing of the software they use from upstream. Red Hat is still not closed source, for instance. Everyone who gets it gets access to the source code. But if Ubuntu went away or whatever then downstream distributions would be in a spot of trouble. They could rebase on Debian (which is what Ubuntu is based upon), but how hard that would be varies wildly depending on distro. Linux Mint already have a Debian edition, for instance. No problem there. Pop OS would certainly be able to make it work as well; they're a very professional operation. But take, for example, Endeavour OS. It's Arch with a graphical installer and some nice defaults. Without Arch Linux (which is almost certainly not going anywhere and is a community distro) they'd have some real problems. There's no upstream to Arch to rebase on. They'd have to so fundamentally change everything to accomodate a whole new base and packaging system that they'd basically be making a whole new distro.