The drafting isn't horrible, they've lacked the picks/have moved all their prospects. Trading Norris and Stutzle was the inflection point. Given the picks in hand, who could they have drafted that would have made a difference these last few years?
Off the top of my head:
One can make a case for Jesper Bratt and Hagel (late-round picks in 2016). Hagel would be useful going forwards (no way the Sharks have the guts/foresight to trade him to Tampa Bay) but isn't a significant pickup; would he have gotten the Balcers treatment given his defensive game? Bratt...I doubt he moves the needle very much in 2020 or 2021 (who knows how he develops on a stacked Sharks team from 2017-2019), but perhaps in 2022 his addition is sufficient to get the Sharks to a playoff spot. Certainly, I'd feel better with him in the top-6 going into the coming season (provided the Sharks could fit his contract).
Outside of that...:
I think Adam Fox was drafted after Gambrell in 2016, but he never would have signed in San Jose. I believe he was eventually traded for a pair of 2nds; not sure that would have netted anything significant.
At this point, I'd definitely prefer Miller over Merkley; maybe Sandin as well.
None of those swaps would have changed a thing about the past few years. Maybe going into next season, I'm a little more optimistic with Miller on the third-pairing...
So, even assuming perfect hindsight WRT drafting...the Sharks are only marginally better, with another top-6 forward, a solid 3rd-player with upside, and a little more security on defense.