The extreme scenario is a new account being started and a challenger player plays on the account.
The account wins the first game and starts with a visual rank in Gold 3. The account goes 20-0 and is playing with D2-Low Masters Players but the visual rank is Emerald 2.
By the time that the visual rank is Masters, then you are already playing in full challenger lobbies.
This is not a made up scenario. Ive been watching AloisNL, former pro player in EU do unranked to masters with every toplaner and Ive been following the accounts. They go 30 wins 4 losses and they always end in Emerald 3 or Emerald 2 but hes playing with 9 Masters players and the series ends.
Now if you want to drag this back to the average player. If your goal is to hit Gold and you start Iron 2. You can play 100 games and win 65 and lose 35 (65% winrate) and your visual rank is Silver 3 or Silver 2 but you are playing vs High Platinum and Low Emerald which is elo way above your target goal.
The system artificially forces you to play between 300-500 games, instead of the 100-200 that you should play in order to reach “your elo”.
MMR is everything, but people play ranked for the visual rank. There is no need to have a mismatch, other than to force players to play 200-300 extra games for no reason in order for them to reach “their true” elo.
What you describe only happens for the people that are smurfing.
For everyone else it is the exact opposite and this is why inflation is a huge thing. Basically a iron 2 player can win 65 and lose 35 and his visual rank will be gold while his mmr is silver. So he is visually inflated to a rank that his skill is not worthy of. And as a result, +15 -30 is what people get. Because the system sees that they are not worth of that skill and tries to bring them down.
That is what is happening right now with ranked and this is why it has become a joke now. Insane devaluation.