The mentality that you have to carry games stems from contempt for your teammates and personal belief that you are superior and thus not in the correct matchmaking pool that SR dictates. You can argue I am generalising for sure but that's where all this bitching arises from. People don't understand they are matched with teammates because those people are similar in ability.
What do you mean by a ladder system? I would describe the current iteration as ladder (and so does the community in general, no?), you win games, you go up, you are matched against tougher opponents.
And yeah I could talk about the SR system for hours but it generally boils down to people blaming their teammates and refusing to concede they could improve themselves, which is just tiring.
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Both are fairly common. Gold and plat boys just love to dick around in chokes 'til the timer runs down lul.