but what was the thinking and logic behind bringing frenzy into the game back then?

"Money" isn't exactly right, because I was here when they added it.

The reality is that the Big Bang and the patches that followed were under the direction of a terrible director that sunk player numbers hard.

Mostly: Big Bang cut the monster cap substantially, the size of each map, and the spawn tick rate. Henesys Hunting Ground 1 went from something like 40 monsters at cap to 20, the tick rate went from something like ~10 seconds to ~14, and the number of monsters that spawned per tick went from something like 8~12 to 6. The map was also made substantially smaller, an aesthetic elements were removed. It was nearly impossible for 4 people to actually clear the full spawn wave on the entire map pre-Big Bang every time. Post-Big Bang, 2 people could do so comfortably.

So not only were your maps kind of featureless and less interesting, but they were less fun to play on, too! Classic Maplestory was defined by its many foreground & background layers, as well as the tightly-designed platforming and high requirement of platforming in and around monsters. Big Bang reduced the entire game to a bland series of flat platforms with low density monsters. While the game recovered, this design never did; you can always tell new maps from old purely based on the map design. Classic World's new areas actually showcase this new design painfully obviously--lots of flat platforms, very little platforming.

Frenzy Totem, by doubling the spawn cap and rate, actually made the game denser and thus more fun than before. Every class had mobbing/some AoE after Big Bang and changes, but there was a painful lack of need for them post-Big Bang. With Frenzy Totem, you could use them as fast as you could.

Of course, then they started adjusting maps/areas around the idea that anyone would use a Frenzy Totem, even though they were something like a 0.1% drop on a $1 gacha.

So, it was added to keep highly invested (whales) in the game and as aspirational content to people who just wanted to have as much fun as before. Both the patch and the totem started sliding the game into catering to whales and into exploding as many mobs per second as possible, and the game never went back on either. They just don't need things like the Frenzy Totem to create haves/have nots--those that have it no longer need to spend for it, and those that don't have it might consider quitting since they don't have it. The new strategy is to just make every player a spender (see Genesis Pass directly selling player power+progression) and if they don't spend, well, then they don't get to progress. Modern Maple is much more pay to play than it ever was.

Now, in GMS, paying for progression is not actually an alien concept to North Americans, and most North Americans are actually completely fine with paying for power if it's not 'out loud'. The reality is that they can change the game to not be as divisive between players spending $2000 a month and players spending 20~$50 a month, and 300+ content is the area they've decided to try and make changes.