The system encourages smurfs though.

Even rank boosting aside, you could say this for literally any competitive game that has the player choose anything that isn't completely symmetrical. Races in StarCraft, heroes in Overwatch or any MOBA, or characters in a fighting game, etc.

Say, for example, someone gets to D1 playing only Assassins. Well, they're probably bad tanks and supports, so they're playing D1 level assassin and maybe Gold or Plat level Tank/Support. So they want to learn another role, what would be better for them: Get carried/lose a bunch of games in their current rank? Or somehow come up with a separate account where you can practice that role?

Because, at least specifically in Overwatch, tons of people make separate accounts for exactly that reason. The ranking system could be the most infallible system in the entire history of ranking systems, but you'd still have issues like that. The system doesn't encourage smurfs for whatever reason you're saying.

Why would you get to GM then stop playing? Like, actually what would the purpose of that be? The game already snapshots your highest rank. If you get master and drop to silver 5 you're still getting Master rewards at the end of the season. By getting to GM and stopping you're doing the exact same thing as getting to GM and losing for eternity. Your rank will only go down because other peoples' points are going up while your points remain the same.