The problem is, the divide between the Haves and the Have-Nots from this expansion is so massive that nerfs alone would not save it. We're talking about multiple classes pulling sub-40% winrates, you'd need to obliterate everything from orbit thrice over to give these classes a chance using only nerfs alone. It's much more practical, and likely to produce results, to push the weak and underperforming archetypes to match the current power level instead of dragging everyone else down to their level.
Also, your example of Wildfire is very flawed because since its buffs, Wildfire is not core to EVERY Mage deck. Spooky, Ping, and some versions of Naga Mage did run Wildfire as a core card, but not Mech or Big Spell, and the latter two were much more popular last expansion.
Why would you not run wildfire in any mage deck other than mech mage??? They do run it in the skeleton mage too! I'm watching a tournament right as I'm typing this where they're running it! They even run the freaking card in big spell mage. And once again, if they nerf cards and the meta's still bad, just nerf more lol. It's that simple. One of HS's problems is that they're too conservative about how they nerf cards now, and they only ever nerf the most problematic things. The Legends of Runeterra balance team is way better because they don't just nerf the best deck. They have the foresight to know that nerfing the best deck makes the 2nd best one the best.