I guess the thought was it was meant to be a single burst at the start of the fight, with a bit of normal damage afterward before a second burst a bit later. It wasn't that much before its buffs, but then the buffs took it to an extreme in an attempt to let crit marksmen keep up with the lethality marksmen at the time of the buffs (Varus, Miss Fortune, and sometimes Jhin if memory serves).
The Rapid Fire charging it faster makes the problem a lot worse as well, since you don't need to kite as hard to keep the burst going, making Runaan's Hurricane and Phantom Dancer inferior choices for a lot of champions who use Shiv.
I don't see what was wrong with it not charging at the rate it did, or what damage it was before (100 base damage at all levels in Seasons 3, 4, and 5, vs 60-160 base damage on Shiv right now, which breaks even at level 10, around when most champs get shiv anyway, and scaling higher afterward).
Before the changes that brought it into the state it's in now, picking up shiv meant you were taking an AOE burst but losing sustained damage (with Runaan's Hurricane or Phantom Dancer) or range (with Rapid Fire) or raw damage (with Essence Reaver or Infinity Edge, depending on if you're a caster or a pure AA champ), or sustain (by picking Botrk or Bloodthirster). But now that Shiv charges faster with Rapid Fire and gives so much raw damage, as well as Fleet Footwork (and maybe bloodline) giving the needed sustain that also charges faster with RFC, you're not losing out on any of those things by picking the two of the items up together. Just pick the right runes and you've gotten everything that used to take four item slots in just two or three.
I'm just glad something's being toned down, but imo it needs to be more than just what they're doing, since what is being given right now with Shiv (and Rapidfire by extension) is too much at once, and it's probably going to stay that way even after the nerfs.