They have demotion protection. It takes a few games to demote. Promo helper gives you free wins next time.
Promos are in place to slow down progress to prevent luck from getting some one to a tier.
This is not to make the climb longer. this is because by separating LP and achieved tiers from MMR you can have several changes that affect player perception. basically much any account with Bad LP gains and high LP losses has a MMR below their current tier. in a Pure MMR your Tier would instantly reflect that by having you be lower.
This is fine for players who are climbing, they instantly are higher each win. But seeing the movement of MMR each game frustrates a ton of players, about half the ladder will only ever Lose MMR. LP system just helps hide the direction players are moving, and as your End Tier is what gets used to represent your "skill" you can have the actual MMR gains and losses per match be higher then a pure MMR system with out it instantly making accounts achieved "skill" a gross misrepresentation impacted by luck. In other words you lower the # of games required to get MM to have a more accurate MMR value for skill on players with out giving them a false concept of it from luck.
You need to have slowdowns in place if you do this. Also if your MMR gains losses are to high MM becomes very rubberbandy. You can get away with a rubberbandy MM system if you use a K factor slowdown that dials down Gains/losses as accounts play games and add datapoints. Ideally youd have multiple slowdowns to really narrow in the players to a accurate range of MMR to Skill expectations. Then its just about waiting for LP to catch up. This looks Bad on MMR so you have to have lower Gains and losses or risk players freaking out not understanding the importance of accounting for account uncertainty. The fact players think high uncertainty accounts are magically accurately placed in the system just because of the claim "make a match on average 50% fair for both teams with usually only 1% variance" is very telling how little most players know about MMR systems. Its a completely misunderstand the end goal, or why Elo had his adjustment formula and system built to be self correcting and objective.
TLDR. Promos are needed for LP system. Removal of them would have to be compensated bymakeing the time to climb ladder higher. If MMR gains remain the same and your remove promos the quick fix is to nerf LP gains and Losses as you have to prevent in accurate achievements off luck for high uncertainty accounts.