You can make them always on still. I've done so ever since their disappearing act started. It's not even much hidden, it's in the "Appearance" setting pane.
Of course we all make mistakes, but anyone who has made this mistake should really fix it!
Classic Macs were designed for the mouse or trackball. Modern Macs are designed for multitouch scrolling. When it's easy to get the scrolling infrastructure on demand, the desktop might not need the same click-first affordances.
It is configurable, right in System Preferences > General. (Or I guess it's "Settings" now on modern systems, don't know what menu it's in there.)
P.S.: seems like the setting still keeps the scroll bar on top of the windows content (e.g. a website), not outside of the content.
Just like iOS7+ it is possible to position and layer interface elements in a way where the visual effects will render a screenshot difficult to read, but in practice the elements are frequently in motion or as you've already pointed out easy to make them move. That motion is what negates the layering problems, thus making visual occlusions rare, short-lived and easily resolved.
There is a certain unreasonableness in ignoring that reality, and also ignoring that there is a user setting to keep a full-sized version of the scroll bars always visible.
This isn't to take away from legitimate criticisms such as the issue with the resize hotbox not being updated to match the more rounded corners, but rather highlight that not all online forum criticisms comes from a bona fide place.
That changes the effort required to show useful information from zero to more than zero. Which, while it not be a great quantitative change, is an enormous qualitative change.
Like Chesterton's Fence, it was there for a reason.
"At last (and at least) we have reclaimed that narrow vertical strip of screen real estate on the screens eastern-most vestige! Now to find a good use for it!"
The true annoyance is that in many cases explicitly enabling them does not restore the original functionality.
1. The trivial ability it is to resolve, and
2. The existence of an easily accessible user setting to enable the behaviour that you desire.
Fundamentally your complaint thus comes down to a gripe that the OS's defaults don't match your completely subjective idea of how just one of many OS elements should work.
Which raises such an interesting question, because of all of the UX behaviours present on macOS - this is your hill?
...extra padding?
*They're the same thickness as Aqua.
This is endemic now. Cinnamon does it by default and I hate it. I only managed a partial fix, and then I had to do more work per-app (especially Firefox) to make them behave.