When Death's Dance was introduced in 5.22, this is what Riot said on its purpose & who it was for:

"Lifesteal as a stat for champions that primarily use abilities has always been an interesting space to work in, but often failed because the 'casters' in question lack the inherent durability to really start draining and instead just die on the spot. Death's Dance instead provides 'pseudo-durability' - just enough to give the user time to get their sustain engine going, but forcing that lifesteal to work overtime so as not to succumb to their accumulated debt. Can you outrace your own death, or will you just dance around it?"

Basically, it was intended buy squishy AD casters a little bit of extra time to actually use the healing to survive if they can respond with enough damage output of their own within that extra time granted to them. Unfortunately, squishy AD casters are not who is buying this item. It is being primarily abused by bruisers who for the most part have tools and items such that they shouldn't even need an item like this. Currently the only truly squishy AD champion that buys this item reasonably often is Graves with the occasional Master Yi or Katarina.