Wait, is that a rule that a team cant trade back to back unprotected firsts? I'm just not following the rationale for the 2019 first already being traded and unprotected
No, but teams generally value picks less that are farther in the future. Relative to the strengths of the drafts, a pick that is for an upcoming draft is worth more than a pick for the same round in a draft that is one or two years away.
The point is, the Sharks did not have the leverage to protect a pick being traded in the 2018 off season all the way up to the 2021 draft. There is no way the Senators would wait that long.
If the Sharks had their 2019 pick, they could have traded that pick and protected it, and the Senators probably would have done it. They didn't have that pick though because they traded it to Buffalo for Evander Kane.