So why does the "pay-2-win" business model exists?

In a way, it's fair because you're getting what you'd paid for; but from my perspective, this is basically the developer trying to commit suicide. I'll admit that it's fun to run around, shitting on people with an overpowered weapon/character; but such entertainment usually fizzles out in a comparatively short timespan as I'm left yearning for something challenging to hunt...But that's the endgame of Pokémon Black 2 and it's normal for an lot of games once you find the strongest weapon in them. To be fair, the timing of getting the game's version of the Master Sword usually coincides with an difficulty spike.

So according to the grapevine (which is an almost dead, but interesting thread at this point): We have this business model inside an competitive environment. There's an gun that one-kills people up for purchase. Cut to an lot of people only using the aforementioned gun. Cue to whoever plays the game for free/basic subscription leaving in droves. The game is now oversaturated with the bling gun and an ghost town of an server, which will eventually shut down because of lost revenue.

I can advocate the "pay-to-play" scheme if the gameplay is good enough; but why would you add in an weapon that's significantly better than all of the free content and hope that everyone will buy it? I know some people will buy it due to chaos theory, while others will just leave and take their money with them, causing an "short-term profit, long-term declining playerbase" effect. Wouldn't it be better to just try to sale cosmetic features instead of indirectly driving your customers away?

Presumably the number of people willing to buy stuff like that is higher than the number of people that'll quit because of it, or at least provides enough revenue to make it worth it.