Are you trolling? I think the answer is obvious: less duplication of efforts may make the linux ecosystem be more lean which may mean more manpower to... bring the year of the linux desktop.
Distro package managers have more than 20 years doing this. If you want to replace them I think there should be fewer drawbacks. Otherwise you are throwing out decades of design, thought, bug fixes for something more problematic.
If the packaging is done by the dev you are assuming that 1% extra effort wont be spent playing chess instead of adding features.
If the packaging is down by an interested party for that distro you are assuming that a person competent to write a package file is even competent to say write an app let alone interested in doing so.
Flatpak sounds like a shitshow, but the not because it doesn't meet your specific requirements.
Arch has aur, arch can sideload rpm/Deb's, Mac has home brew, who needs this?
The only people I see using flatpak is the 50 odd Linux users we have at work, they constantly have dramas trying to make things work around home directory sandboxing, permissions, updating and more
> Single way
Why is this valuable? Just saying it doesn't make it true.
Why do we need a single way to install?
I'm not saying it's a bad thing, but giving up home dirs and a bunch of other platform specific benefits is a cost I'm not willing to pay for some made up ideal that gives me a worse experience.