I think for something to be a slippery slope it has to be historically/statistically accurate... Major games in the past have implemented micro-transactions, made the same claims as Bungie, and then went back on their promises and were made P2W. There seems to be a strong correlation between games implementing them and the game losing its player base because of said implementation. That's why people are so worried. Because, micro-transactions are KNOWN to destroy the player base of certain games that use them incorrectly. The addition of micro-transactions in gaming has been a heavily debating topic for a very long time which is why the "slippery-slope" argument applies.
As for you're "counter-argument," I've never seen anyone argue either of those things.
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1st argument is the NRA's historical counter to attempts at national gun registration laws, since "the government's first step before confiscating guns would have to be learning who has them" (see the attached scene from "Red Dawn" (1984) where the invaders get gun registration records courtesy of the sporting goods store and go to round up the owner's weapons- classic right-wing fear-mongering there); the 2nd is shorthand for the argument against legalizing pot, since it's a "gateway drug" which leads 'inevitably' to the user trying harder drugs. I've heard both repeatedly over the last 30 years. Then again, we may travel in different circles.