I think I understand both sides - reusing one of opened tabs could be confusing and unexpected, but on the other hand flows described in this thread are also very useful.
What about keeping the new behavior and providing new feature, something like “link tabs”, but where I would specify the target tab, where Obsidian should open links from the specified pinned tab?
So for example I will open 2 tabs, put each next to each other in the window, left is pinned, another is unpinned. And then I right click on the pinned one and select “Set link target panel” (or something like that). Now I will select the second tab as the target. And from now on, all links clicked in left panel will be opened in the right panel. This could be indicated by some icon on the panels, similar to when they are “linked”.
I believe this handles old workflows and also opens up a possibility to new workflows to use.
And perhaps this doesn’t need to be forced to “pinned” and “unpinned” tabs only, but could work universally. So I can even have 2 pinned tabs, and set 1st one to open links in the 2nd one. Where the 2nd one will open its links normally in new tabs. This could be really flexible.