All of that is good but you are overlooking the small detail that installing flatpack implies using up a lot more disk space than just pulling a distro package.
I can point one specific example with libre office: 3.9GB for the pack vs 785MB for the .deb.
We can argue disk space nowadays is cheap but overloading a machine with duplicated packages also goes against the main goal of running a Linux.
When I first started using it, one of the talking points was that Linux kept the system clean of clutter and that improved longevity for the hardware and delivered stability by not having unnecessary and unused or orphaned and redundant libraries and dependencies.
With flatpacks we get the latest and greatest - I'm a debian fan and I hurt for not getting more up to date software - but we are carting in a ton of junk that should not be necessary.
And the container/sandbox part is not that great, apparently. Debian wiki links to this to further educate/alert on the down sides of flatpacks. Debian is not the ultimate bearer of truth but they do move a lot of respect.