The problem is promotion games make you stuck and hinder your ability to rank up and it is not necessary in an MMR based rank system.

Those are some links that explain this problem better than me:

1.  https://boards.na.leagueoflegends.com/en/c/gameplay-balance/EHBEvZw2-remove-promos
2. https://boards.na.leagueoflegends.com/en/c/gameplay-balance/XtqYJIlG-the-ranking-system-is-broken
3. https://boards.na.leagueoflegends.com/en/c/gameplay-balance/8oGApjB1-games-feel-more-coin-flippy-than-ever
4. https://boards.na.leagueoflegends.com/en/c/general-discussion/O6qyMog5-failed-my-promo-series-twice-now-with-zero-losses
   
   
   Remove promotional games. Let people rank up when they reach 100LP
   
   Whenever I reach my promo games I lose and I am stuck because of that. I win and win but I lose my promos and it is not fair in an MMR based rank system that I have to win promotional games when I already got the 100LP.