Soloing back a single expansion has never been super easy in base gear? Not sure why you are surprised.
The main reason it was possible in Legion / BfA was that later on in the expansion additional power systems like legendaries, artifacts, and azerite made our characters stupidly overpowered compared to a fresh max character.
It’s been this way since the dawn of time. Secondary stat ratings change as you level up, you got from having the best gear of the expansion to the worst.
From secondaries at 40%+ backdown to ~10%.
Even more so from expansions that had things like azerite, or essences contributing to large percentages of the damage we dealth (and survivability)