I only see dropping a Tier being the real issue, like from gold to silver, but you're supposed to get a warning notification when you're about to reach that threshold, so you have forewarning. This also only happens when your MMR actually drops below the tiers MMR, so you don't actually DESERVE to be in that Tier anymore. Before, when there was no LP system and it was all based on MMR, you could go up high very fast, but drop SUPER hard as well when you lost, so at least there are gating mechanisms to prevent a super hard drop unless you actually deserve it. As for dropping divisions, I feel there isn't any real need for it because if you play enough, you will hit your "ideal" spot eventually.
Going up divisions/tiers is annoying enough, but I feel it will be even worse if a demotion series existed. To stay in your current tier you will be more prone to rage since you have something to lose in a stretch of a few games called a "demotion" series, which is pretty depressing. At least with going up, all that really happens when you lose is you just need to start over again from scratch, whereas a demotion series if you lose you go down to you lower tier and have to work yourself back up.
