That's a ridiculous answer. "You don't" means people go back to windows or Mac OS where they can do that easily because it's an actual need people have. That's the worst outcome by far as it leads to less support on Linux.
No, "you don't" is a valid answer. You use what is in the distribution. You chose the combination of archives (stable, stable+backports, and so on...) as you need.
Then you use what the distro provides.
(The same way as you use the default, tested and guaranteed engine computer on your car rather than installing one bought in a dark alley.)
Like the car, distribution work when you use the whole set of components bundled and tested together.
If upstreams make it difficult to package new version you can help by asking them to step up their game or switch to a better software.
If you want to build a Frankenstein bundle of software you are free to do so but you'll be on your own when things start to break.
This is absurd. You are confusing Linux distributions with DRM.
Did you not read the last line? You are free to do whatever change you want.
You are also free to use a distribution as it is without having to spend your time handling security, stability and compatibility across applications all by yourself.
Yet you cannot mash together a frankenstein OS while also expecting it to provide security and stability. Not because of a distribution decision but because of reality.
Exactly: There isn't.
> How do I install an older or newer inkscape / ardour / krita in the latest debian stable / ubuntu / fedora / whatever without recompiling stuff ?
You don't. Or if you have to, do it on Gobo (or perhaps Guix or NixOS?).