Quoting: phil995511

"Linux has, historically, been quite messy when it comes to software distribution. We have various distribution-specific packaging formats like .deb (Debian / Ubuntu), .rpm (Fedora) and the list just goes on and on. Canonical also went their own way with Ubuntu for the likes of Snap, but the real winner is going to end up being Flatpak and the main Flathub store."

Software distribution under Linux is anything but chaotic. On the contrary, it is extremely orderly.Each distribution offers its own software library and package format. This, among other things, increases system security. This makes it much more difficult to implant malware under Linux.

Snap was most certainly developed by Ubuntu to try to provide unique packages for all variants of its distributions, thus avoiding having to recompile packages x times for each of the Ubuntu versions still in use.

Flatpack will never replace .deb, .rpm, or other packages. Furthermore, unverified Flatpacks can pose serious security problems on a system.

Flatpack is used as the primary source of software access on distributions at the stage of development alpha / beta, on distributions that are immutable due to their structural complexity, or on new distributions that do not yet have enough software to offer their users. Flatpack is in no way intended to replace .deb, .rpm or other formats.

To understand Linux, analyze all of this from the perspective of developers and system engineers/architects. As an amateur unfamiliar with Linux, it is impossible to speculate on this subject without making a mistake.

From a down-to-earth point of view, Linux is free and benefits large groups such as IBM, Oracle, Microsoft, Amazon, Google, etc.

It's not Valve and its Steam client that are giving Linux a boost, but rather the other way around. Linux is provided to Valve in a fully functional state. Valve then simply adds its own layer (just like with Android for telephony), allowing them to launch an operating system bearing their name while drastically reducing research and development costs.

Today Linux is much easier to access for inexperienced users, but it should not be forgotten that this is an operating system developed by and for experienced professionals.

A. Liam writes to new users or at least not kernel developer level users
B. As the history of appimage and android APK's clearly displays those large players you mentioned would've loved a neatly unified approach to package distribution.
C. The third software freedom is formulated by the fsf as and I quote here: "The freedom to distribute copies of your modified versions to others (freedom 3). By doing this you can give the whole community a chance to benefit from your changes. Access to the source code is a precondition for this." Clarifying here clearly that shared by those experts you mentioned aren't only meant for themselves, but for the entire community. You might argue that only the developers qualify under the community, but that isn't how I see it used generally. Most of the time it's used to describe all users. The fsf is a group of those contributing experts. This implies that at least some of these experts don't develop it just for experts.
D. The beauty of open source is that it's the tide that lifts all boats. As such Valve indeed gets to lift on the works of others with Linux and the ecosystem. Yet, it also adds to Linux and the ecosystem by submitting its own patches. These patches add value for many beyond Valve. For these patches I'm thankful towards Valve even though they as such benefit from community support and R&D savings. They drastically improved Wine, gave KDE some neat extras, solved several driver issues and enabled equal graphics processing with Windows through their contributions to the Khronos project.
E. Your Snap analysis shows, why Flatpack has massive value for those experienced professionals. They don't want to compile their programs a billion times for a billion different distros and versions. Snap offered insufficient room for competition and customization as such faltpack was introduced.
F. Unverified flatpacks aren't half as dangerous as unverified packages(yes, you can download and install debs straight from the web), since these have full root access. Unverified is just dangerous.

Having said that:
A. You're completely right .deb/.rdp/.snap aren't going away, but they'll be brought back to the place they belong: root level system extensions like drivers, virus scanners and such. I still get shivers every time I install a game or a production program through apt and it requires root.
B. Yes, it makes malware spreading harder, because it makes software distribution generally harder. That's a bug not a feature.
C. Flathub(not Flatpack) does add a level of insecurity Linux didn't previously have. It has a pretty loose security policy. Most repos including the fedora flatpack repository require the repo maintainers to have compiled and understood the code before pushing it(canonicals snap store is an exception). Flathub doesn't. This is dangerous as Canonical has displayed with several cryptoscam wallets in their snap store.
Especially and this probably what you meant with the danger of unverified Flatpacks, because flathub does make it dangerously easy to install non-developer signed(unverified) Flatpacks. Most repos get away with providing non-developer signed packages, because they can check them for themselves. Flathub doesn't do that.